Elisabeth C. Miller Library

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Plant Answer Line Question

Keywords: Mulching, Compost

Can you give me some good sources for information about mulching and different mulching materials?


Below are many links to information about mulch, including several from Pacific Northwest government agencies. Explore these sites for lots of other useful information about gardening! There are also many helpful books on the subject, such as Mulch It! by Stu Campbell (Storey Books, 2001) and the Brooklyn Botanic Garden's guide, Healthy Soils for Sustainable Gardens edited by Niall Dunne (2009).

There is an excellent introductory article on the Brooklyn Botanical Garden website entitled "How to Use Mulch".

ABOUT MULCH, types, and uses--Cornell Cooperative Extension (NY)

King County (Washington) Solid Waste Division mulch info
Make the mulch of it!

-- Saving Water Partnership (Seattle)
--King Conservation District (Washington), manure share program


Date 2019-10-03
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Plant Answer Line Question

Keywords: Arctostaphylos, Native plant gardening, Mulching

Is is good to mulch Arctostaphylos uva-ursi? If so, would an aged bark be best or a mulch that contains manure? How deep should the mulch be?


Native Plants in the Coastal Garden (by April Pettinger, 2002, p. 27), says the following about mulching Pacific Northwest native plants:
"...When an established native plant garden requires maintenance, it is usually minimal: mulching is probably the most important---and often the only---maintenance required. In any garden, mulching is arguably the most beneficial care you can give your soil and your plants. There are many advantages to using mulch. It suppresses weeds, conserves moisture by minimizing evaporation, and releases nutrients to the soil...Good mulch materials are compost, decaying leaves, well-rotted manures, sea kelp, mushroom compost, seedless hay or straw, shredded prunings, natural wood chips, grass clippings and evergreen needles and cones. Commercially available screened bark---usually referred to as bark mulch---has little to offer other than its ability to conserve water; it has no nutritional value and in fact depletes the nitrogen in the soil. When spreading mulch, don't pile it too close to stems of plants. If you are using compost as mulch, spread it about 2 to 4 inches deep. Other materials may be applied to a depth of 3 to 7 inches..."

Date 2019-10-04
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Plant Answer Line Question

Keywords: Soils, Soil amendments, Mulching, Compost

Help! My clay soil is stunting the growth of my plants. I've amended the soil with compost and manure. Is there a another method of conditioning the soil that you can recommend?


First and most important, it appears mulching is the best organic solution for conditioning clay and heavy soils. Organic soil conditioners include compost, well-rotted animal manures, and natural fertilizers. Planting green manures such as clover, rye grass, or vetch are also effective for breaking up large clods in clay soil over time.

Sheet composting - laying compost over the entire area to be worked and using a fork (or rototiller) to work it into the soil to a depth of 2-4 inches - is cited by the resources listed below as an efficient method of soil conditioning. Both books listed below recommend repeating this process at least twice a year, in early spring and in late fall.

Secrets to Great Soil [by Elizabeth P. Stell, 1998, (pbk)] and
The Gardener's Guide to Better Soil [by Gene Logsdon, 1975, (pbk)]

The Saving Water Partnership (the City of Seattle and other government entities) has a website full of information about improving soil.

The site includes Growing Healthy Soil.

Current thinking contradicts the notion of working compost or other amendments into the soil, as explained in a March 31, 2010 Garden Professors blog post by Professor Linda Chalker-Scott of Washington State University Extension Horticulture. She specifically takes issue with the "Growing Healthy Soil" information linked above. Here is an excerpt:
"Not only will extensive digging or rototilling destroy any soil structure you might have, it will also take out the roots of any desirable plants in the vicinity). [...] improper soil amendment can cause serious problems such as soil subsidence, perched water tables, and nutrient overloads. This last point is especially important to anyone living near aquatic ecosystems, since excess nutrients always end up in the water. Before you plant this year, find out what your soil needs before amending it. And remember that mulching is the natural (and sustainable) way to add organic matter to the soil."

Date 2019-08-14
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Plant Answer Line Question

Keywords: Weed control, Mulching

What is the best time of year to use Casoron and/or Preen for weed control on ornamental beds?


Both of these herbicides are registered pesticides, and the law requires that they be used in strict accordance with the directions (and only on the weeds/pests for which they are registered). It is safer for you and the environment if you manage weed problems without the use of pesticides.

You may wish to know more about these particular pesticides. Both Casoron and Preen are pre-emergents, meaning that they work to kill seedlings before they sprout. This means they will not eliminate weeds that have already broken through the soil surface and are growing above ground.

Casoron is persistent in both soil and water (i.e., it hangs around). Its active ingredient is dichlobenil. There are numerous environmental and health concerns associated with this chemical. Dichlobenil will kill any plants which are exposed to it, and will harm beneficial soil microorganisms. Below is a fact sheet about dichlobenil from Northwest Coalition for Alternatives to Pesticides.

The active ingredient in Preen is trifluralin. It is a suspected carcinogen, and is toxic to fish and aquatic life, and earthworms. Here is more information from Cornell University and Extension Toxicology Network UK.

The links below provide information about alternatives to chemicals for weed control. Northwest Coalition for Alternatives to Pesticides has a page of factsheets about specific weeds and ways to manage them. Here is their page on managing weeds in garden beds.

Toxic-Free Future (formerly known as Washington Toxics Coalition) also has information on ways to handle weeds in the garden. Here is more information in a PDF file.

Before reaching for chemical weed control, it makes sense to adopt gardening practices which will help keep the weed population low. Mulch is an excellent way to control garden weeds. After you manually remove weeds from an area of your garden, apply a layer of mulch. This should suppress weed growth and help retain soil moisture. Here is what garden expert Cass Turnbull says about mulch:

"Not only does mulch retain water, smother tiny weeds and weed seeds, and make it easy to pull new weeds, it is also harder for new wind-borne weed seeds to get a foothold.

"Mulch can be spread anywhere from 1 inch to 4 inches thick. The thicker it is, the more effective and longer lasting. Spread it thick in big empty spaces. Spread it thin around the root zones of shrubs to allow for sufficient air exchange, especially around shallow-rooted plants like azaleas and rhododendrons. And never let mulch stay mounded up in the base or the "crown" of a plant. It can cause crown rot on some shrubs and can kill them, even a year or more later."

Source: The Complete Guide to Landscape Design, Renovation, and Maintenance, Betterway Publications,1991.

Date 2019-10-04
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Plant Answer Line Question

Keywords: Mulching, Asparagus

Can cotton hulls be used as mulch on asparagus?


Here is what Stu Campbell's book, Mulch It! (Storey Books, 2001) says about cottonseed hulls as mulch:

These hulls can be used most effectively around plants such as beans, which are suited to wide-row planting. Apply a 1- to 2-inch layer. Or you can wait until the plants have grown 3 or 4 inches high, then sift the mulch down through the leaves... Cottonseed hulls have a fertilizer value similar to, though not as rich as, cottonseed meal. Because they are so light, the hulls blow around in the wind.

Campbell discusses mulching asparagus with a choice of hay, leaves, straw, old manure, and compost for winter protection. You can leave these mulches in the spring, and the tips will emerge through the mulch. If you wish to extend your growing season, he recommends dividing your bed in 2 parts in spring. Mulch one half heavily with fine material like cocoa hulls, leaf mold, or ground corncobs. Leave the other half unmulched until shoots break through the mulched side. Then mulch the unmulched side. The half which was mulched earlier will bear a few weeks later than the other half.

The National Sustainable Agriculture Information Service has a web page on organic growing of asparagus crops which mentions other types of mulch (such as winter rye as a dying mulch and perennial ryegrass and Dutch white clover as living mulches) for this crop, applied at different times.

It sounds to me as if the main drawback with cottonseed hulls is their light weight. Otherwise, they should be acceptable as a spring mulch.

Date 2019-10-30
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Plant Answer Line Question

Keywords: Weed control, Rosaceae (Rose Family), Mulching

We recently had a large blue spruce tree cut down and had the stump ground. Would the resulting sawdust be a good mulch for roses?

Also, what is your opinion of using a pre-emergent herbicide for weed control in our rose bed?


Sawdust has high carbon content and may rob soil of nitrogen and moisture. It is also recommended for acid-loving plants and may be problematic for roses. There may also be compaction problems with sawdust, so it may need to be combined with other mulching materials to improve water penetration. Sawdust also decomposes slowly and compacts (Source: Mulch It! by Stu Campbell, Storey Communications Inc., 2001).

You may also be interested in two articles by Linda Chalker-Scott. In Wood chip mulch: Landscape boon or bane, she discusses the pros and cons of wood chip mulch. She also comments on sawdust in an article called The Myth of Pretty Mulch.

If you had spruce chips, they would be fine for mulching roses. Avoid letting mulch touch the main stem; the goal is to pile it on the root system away from the stem. You can remove it in the spring, or at least be sure that it's not too deep. While mulch protects from cold in the winter and drought in the summer, if it's too deep, water cannot get to the root zone of the plant.

I would recommend that you avoid chemicals, as I find that you have to pay more attention when you use them than if you just wander through the garden now and then and pull all the weeds you see.

Pre-emergent weed controls never provide complete weed control. The most important thing to do is weed the area first, as pre-emergents only control weeds that have NOT sprouted. And if you have lots of seeds in the soil, don't expect weed killer to eliminate them all. If water is required, beware of too much water (i.e., rain) that can wash away the herbicide.

Rather than use a chemical, I would weed the area now and then apply mulch. In addition to protecting the roots and soil, the mulch will suppress weeds, possibly until spring. You will have to watch for weeds that do sprout and be sure that you don't let them go to seed. Otherwise, you will set yourself up for lots of future weeding. Chemicals don't really help in situations like that, as you have to time their application perfectly. Hand weeding and mulching--well timed--can work better than any herbicide.

Date 2019-05-16
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Plant Answer Line Question

Keywords: Mulching, Iris

I covered my iris bed with wood chips to keep the ground from drying out so fast. Since the rhizomes grow partly above ground, will being covered with chips harm the plants?


Your mulch is unlikely to harm your bearded (rhizomatous) irises, as long as it is less than one inch thick over the rhizome tops and allows air through. Here is some information from The Gardener's Iris Book by William Shear: "How deep should the rhizome be set? That depends. In light-textured soils, it can be covered by as much as one inch of soil, but for average to heavier soils, the top of the rhizome is best left exposed to the healthful influences of sun and air. Remember that the rhizome is a stem, not a root, and needs to breathe!...If you do apply a mulch, it must be loose and airy, so it won't pack down and get soggy--a sure ticket to rotting rhizomes in the spring." Shear suggests pulling back the mulch in spring (since it is for freeze protection in his mind), but it seems to me that would defeat your purpose. You might meet both goals (moist soil and dry rhizome tops) by mulching around the irises but not right on them.

Date 2019-12-27
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Plant Answer Line Question

Keywords: Wood chips, Mulching, Allelopathy

We've taken down some big cedars and chipped the branches. I've heard that cedar mulch can damage plants. What is your take on this? I already put it around some choice pines and some viburnums, but I could move it if need be.


Washington State University Extension horticulturist Linda Chalker-Scott has written about this very issue, and her scientific research on the subject says that cedar (both Thuja and true Cedrus) wood chips are not allelopathic (toxic) to plant tissue. Here is the article.
This author has further information on the general benefits of wood chips as mulch. Here is a newer Washington State University factsheet on the subject.

Date 2019-10-31
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Plant Answer Line Question

Keywords: Mulching

Are pine needles good for mulch in the garden and for fruit trees?


Pine needles are most often used with plants that prefer acid soil, but according to Mulch It! by Stu Campbell (Storey, 2001), they can be used elsewhere, too, at a depth of one to one and a half inches. It is considered a myth (see information from Washington State University horticulture professor Linda Chalker-Scott in Organic Gardening, February 2007, p.8) that they will acidify your soil. They are good for insulation, weed control, and moisture retention, and excellent for water penetration. However, they decompose very slowly and are not liked by earthworms. You can shred the needles a bit before applying them as mulch, and that will speed their decomposition.

Date 2019-10-02
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Plant Answer Line Question

Keywords: Oryza, Mulching, Compost

Can rice straw be composted or used as mulch, or is there a risk of it sprouting? I have rice straw wattles that were used for an erosion and stormwater runoff control project which is now complete, and I'd like to use the straw in them.


I checked a couple of our books on grain growing, Homegrown Whole Grains by Sara Pitzer (Storey, 2009) and Small-Scale Grain Raising by Gene Logsdon (Rodale Press, 1977). Both of them refer to using rice straw as a mulch, Logsdon saying it makes an excellent mulch and Pitzer saying it can be left in the field to enrich the soil after the grain is harvested. In general, composting books don't make any distinction between different types of straw, but caution against the use of hay because it may contain more seeds.

I don't know which rice wattle product you used in your project, but some of them claim to be weed-free. You would certainly need to liberate the straw from the binding material (plastic netting or geotextile) which gives it its wattle shape before composting or mulching it. Another consideration is the source of the rice straw: if you are gardening organically, you will want to be sure that the rice was not treated with pesticides.

If you are curious about the nutrient balance in rice straw, University of California, Davis's publication, "Rice Producers' Guide to Marketing Straw," October 2010, includes discussion of this issue.

Date 2019-10-31
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Plant Answer Line Question

Keywords: soil contamination, Mulching

We are planning to put down a seasonal mulch this spring and fall, and wondered what your opinion is of colored bark. Would brown, black, red, or some other color be best in our ornamental beds?


May I answer none of the above? There are better options. I can't think of any good reason to apply mulches which have added dyes and colorants to any landscape. Check the Material Safety Data Sheets for each product to verify the source of the dye to be sure that they are not going to cause harm (to the landscape and the people and animals in it, and to stormwater). Some packaged colored mulches also have herbicides added, and I would highly recommend not using them. The sources of the "bark" may be an even more important cause for concern. This information from University of Massachusetts Extension points out that dyed bark mulch made from recycled treated wood can introduce toxic substances you would not want in your garden.

Although this is an aesthetic judgment call, to my eye, colored mulch always looks unnatural in the landscape compared to materials such as compost, leaf mulch, and arborist wood chips. Washington State University Extension professor of horticulture Linda Chalker-Scott is a strong advocate for the use of wood chips as mulch. My own observation is that bark mulch (in general, not just the colored bark products) often introduces weeds into a landscape. Chalker-Scott supports this observation in her book, The Informed Gardener (University of Washington Press, 2008):
"I have seen a number of landscaped sites where applied bark mulch immediately gave birth to horsetail seedlings."

She further states that bark mulch from trees which have been kept in salt water can increase salt levels in your soil. Tree bark has a waxy covering, so bark mulch is not the best choice for absorbing and releasing water.

To summarize, unless you have your heart set on the look of dyed bark mulch, your garden beds would benefit from the alternative mulch materials (compost, leaf mulch, free arborist wood chips) mentioned above, plus planting an appealing and naturally colorful selection of ground cover plants in areas where that is possible.

Date 2020-07-08
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Plant Answer Line Question

Keywords: Organic gardening, Mulching

While researching in the Library's periodicals yesterday, I read an article in Heirloom Gardener about getting tomatoes to ripen earlier. One of the steps was to put down a thick layer of newspapers, which would decompose and later be turned in to the soil. I am concerned about this being organic. However, the Internet sites I found were divided depending on the type of ink used. They made no mention of bleach or other chemicals used in production of the paper, but I wonder about that too. Finally, not many seemed very up to date.

Can you find better information?


There is a question like yours answered by George Weigel in PennLive.com, in which he suggests that it is probably relatively safe, given that many newspapers are now printed using soy-based inks. Here is an excerpt:
"That doesn't mean there still aren't people recommending against newsprint for various reasons (i.e. 'What about the waxes, pigments and other additives that might be in soy ink?' 'Aren't a majority of soybeans genetically modified, so doesn't that taint soy ink as a natural product?' And 'How do we know for sure that someone didn't slip something toxic into a batch of ink or that the newspaper temporarily switched to questionable ink because it found a bargain somewhere?')
"I guess you could argue that newsprint ink might not be 'safe' for those kinds of reasons, but then you could argue that just about anything in gardening poses a threat (what's in the water you're using, your fertilizer, fungicide-treated seeds, genetically modified corn varieties, pathogens in the compost, even the air you're breathing while putting down your newspaper mulch)."

Like you, I've wondered about newspapers as mulch, or as a shredded addition to the worm bin, too. One could not say that the papers and their inks are "organic," but most sources (like the one above) seem to say that the amount of toxicity that might still be present is small compared to other sources of toxins in our environment.

As far as use of newspaper in organic gardens, the Organic Materials Review Institute (which lists what is and is not allowed in certified-organic growing) covers this:

Newspaper is "allowed with restrictions" when used for pest, weed, or disease control, and is classified as a synthetic (not organic) control:
Class: Crop Pest, Weed, and Disease Control Origin: Synthetic Description: Glossy paper and colored inks are prohibited. Paper may only be used as a mulch or compost feedstock.
NOP Rule: 205.601(b)(2)(i) & 205.601(c) As herbicides, weed barriers, as applicable: Mulches. As compost feedstocks: Newspaper or other recycled paper, without glossy or colored inks.

I definitely recommend removing any colored newsprint and glossy inserts that come with the average daily paper. I personally wouldn't use newspaper mulch in a bed where food is being grown, but perhaps I am exceedingly cautious. You might want to be aware that nanotechnology is now being used in some printing inks, and in some glossy ads (such as Macy's) which are scent-microencapsulated (I found out about this because I complained to Seattle Times management about the odor). This link to Ink World magazine discusses the use of nanotechnology in printing. Harvard's School of Public Health explores the environmental and human health implications of nanotechnology in printing.

Grist Magazine has also addressed the related issue of using newspaper in compost.

Date 2019-08-07
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Plant Answer Line Question

Keywords: Mulching, Fertilizers, Compost

I have a large and never-fed garden. I wish to start feeding these plants. I'm composting now, but when and how often do I fertilize?


Has your garden been thriving in years past? If so, I don't think you need to add fertilizers. There's no single packaged fertilizer that will be universally beneficial to every plant in your garden. Different plants have different needs, and it's never wise to add fertilizer without doing a soil test.

On the other hand, you mention you are tending a compost pile. You can apply compost once or twice a year and it will be helpful to all your plants.

Here is information from The Ann Lovejoy Handbook of Northwest Gardening (Sasquatch Press, 2007):
"When do we need to feed? For ornamental plants, including trees and shrubs, spring and fall are the traditional feeding times. Spring feeds are generally fast-acting, offering rapidly growing plants the nutrients they need for a strong summer performance. A feeding mulch of compost can be fortified with fast-acting alfalfa, which will release more nitrogen if combined with composted manures. Alfalfa comes in meal or pellets. I like the big pellets used to feed goats, which are easy to spread and are available without added medication [my note: some alfalfa is treated, for use as animal feed].

"In fall, most plants stop producing fresh top growth, even though our Northwest winters are generally mild. Fall is a good time to feed roots, which continue to stretch and grow underground despite low temperatures. Adding whole fish meal to your compost feeding mulch will fortify roots with phosphorus."

Professor Sarah Reichard, director of UW Botanic Gardens, discusses fertilizers as sources of pollution in our water supply. In her book The Conscientious Gardener: Cultivating a Garden Ethic (University of California Press, 2011), she states:
"If water inevitably drains downhill, how do we stop fertilizers and pesticides from moving with it? The easiest way is not to use them. This needn't come at the cost of your plants. Most woody plants and herbaceous perennials do not require much fertilizer. Mulching with well-aged manure, compost, or other easily broken-down organic materials will supply all the necessary nutrients."

Date 2019-09-11
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Plant Answer Line Question

Keywords: cork, Quercus suber, Mulching, Container gardening

I attended a garden lecture where the speaker recommended using wine corks in the garden, either ground up as a mulch, or whole in containers. She said cork would help with moisture retention in the soil. Do you agree?


The primary source of wine corks is the cork oak tree, Quercus suber. The species name is a clue to the fact that cork is largely made of suberin, a waxy hydrophobic (water-repelling) substance found in other woody plants.

It does not make sense to use a hydrophobic substance as a mulch, since mulch is meant to allow moisture to reach the roots of your plants, not to repel water. For the same reason, it does not make sense to add cork to your containers, either. Professor Linda Chalker-Scott of Washington State University says that cork will not in any way help the soil retain water.

Chalker-Scott has long advocated using arborists' wood chips as a mulch rather than bark mulch, for similar reasons:
"Bark does not function like wood chips in its water holding capacity. Bark is the outer covering of the tree and is heavily suberized to prevent water loss. Suberin is a waxy substance that will repel water, and in fact helps explain why fresh bark mulch always seems dry. Wood chips, on the other hand, consist primarily of the inner wood, which is not suberized and has the capacity to absorb and hold moisture."

Date 2019-09-11
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Plant Answer Line Question

Keywords: Mulching

Last fall, I raked up my fallen cherry tree leaves, and put them in my vegetable garden bed to use as leaf mulch. I don't know what our cherry tree species is, except that it appears to be ornamental. Then I covered the leaves with burlap coffee bags to winterize the garden bed.

I've recently heard that cherry tree foliage and twigs release cyanide when they wilt and decompose. Does this mean that there is cyanide in my garden soil? And if so, will the cyanide be transferred to any vegetables that grow in it? Safe, or unsafe?


A general rule of thumb with fruit trees (in case your cherry is an edible cherry variety) is to gather up fallen leaves under fruiting trees and remove them for good garden hygiene (preventing the spread of disease and any unwanted insects). Provided your cherry is healthy, I don't think the leaves would pose a serious problem if used as leaf mulch on top of the vegetable garden beds.

The leathery evergreen leaves of English laurel (Prunus laurocerasus, same family as deciduous cherries and plums) are not a good choice for the compost pile because they take a very long time to decompose (which is not the case with deciduous cherry), and they contain cyanide compounds. However, so do apples, plums, almonds, peaches, apricots--all have some cyanide in them.

This link briefly mentions the question of cherry leaves and cyanide in terms of toxicity to animals. Fresh new leaves have a higher concentration of hydrogen cyanide; toxic potential is gone when the leaves turn brown. True enough, this is about eating the leaves, not letting them sit on the soil, but I imagine the concentration would be very low, and there are all sorts of things in our soil that in small doses are not likely to cause harm, and may or may not even be taken up into the roots of anything you plant there.

I asked Washington State University Extension Horticulture professor Linda Chalker-Scott what she thought about the safety of using those leaves and twigs on your vegetable bed, and she said, "Cyanide does not persist in soils. It is HCN [Hydrogen cyanide], which is quickly broken down by microbes in search of nitrogen. It is really nothing to worry about.

Date 2019-12-07
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Plant Answer Line Question

Keywords: Trees--Care and maintenance, Shrubs--Care and maintenance, Mulching, Leaves

I have heard that raking to clean up leaves in the fall is no longer a recommended practice, and that it is more sustainable to leave the leaves alone, to decompose in place. Can you provide information supporting this?


This is a complex issue without a single simple answer. Letting leaves lie in place is fine in some situations and not in others. Undisturbed leaves can be a beneficial mulch in the garden, but not all leaves, and not in all circumstances.

Here are a few suggestions:

    Don't leave diseased or insect-affected leaves on the ground. (As a precaution, I always rake up and remove leaves of fruit trees.)
    Be aware that a thick mat of leaves can make a great hiding place for slugs and snails, and early stages of stink bugs. Learn to recognize their pearly little eggs!
    If you have thick or leathery leaves (such as oak and magnolia), a mulching mower might be a good way of breaking them down to use as mulch.
    If leaves are slippery or are going to obscure uneven terrain that might cause people to trip or fall, rake them.
    If you do decide to collect leaves into a pile, raking is a better method than using a leaf blower (and if you must blow, use an electric or battery-operated blower, which is still noise pollution but at least not air pollution).

In their 2014 book The Living Landscape: Designing for Beauty and Biodiversity in the Home Garden (page 146), Rick Darke and Doug Tallamy do advocate letting leaves lie in the garden, but they recommend running over them with a mower first if you want to have finer material for mulching (so as not to smother fragile plants)—which means they would need to be raked into a mowable pile first. Another way to make leaves more useful as mulch is to compost them for a year—which would also require raking them up and putting them in a pile. Generally, though, the authors favor the let-it-lie philosophy: "In many places, the most conserving, functional, low-maintenance approach to autumn leaves is to let them lie where they fall. This is exactly what happens in unmanaged forest ecosystems. Though a too-heavy leaf layer can have a negative effect on delicate herbaceous plants, most trees and shrubs and a wide range of sturdy perennial herbs grow well under natural leaf fall."

I will point out that a tiny urban garden is quite different from an unmanaged forest ecosystem, so sometimes we need to rake leaves where they impede access to paths, steps, sidewalks, etc. And again, consideration should be given to preventive measures. Don't let lie the leaves of any plant you know to be struggling with pests and diseases.

Date 2019-11-14
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Plant Answer Line Question

Keywords: Seaweed, Myriophyllum spicatum, Mulching

I noticed a Seattle P-Patch garden that was using milfoil from Lake Washington as a mulch in the vegetable beds. That made me wonder about using other aquatic plants as mulch, such as seaweed. Would this be beneficial to the plants? Or would it add salt to the soil and cause problems?


Milfoil (Eurasian watermilfoil, or Myriophyllum spicatum) is a Class B noxious weed in Washington State, and it is on the quarantine list. I am not sure whether moving milfoil dredged from the lake into a garden as mulch violates the quarantine’s prohibition on 'transport of plants,' but presumably it had died back before being spread on the beds. When the plants decay, they do impart nutrients (potentially beneficial to the soil, but a detriment to the lake because they cause algae growth), but Lake Washington is not a pristine body of water, and I would be somewhat concerned about pollutants.

As for using seaweed as mulch in the garden, the book Seaweeds: Edible, Available & Sustainable by Ole Mouritsen (University of Chicago Press, 2013) notes that seaweed has been used as fertilizer for centuries in coastal regions. "In France and on Iceland, this practice goes back at least as far as the 14th century." In Scotland and Ireland particularly, scraps of seaweed that wash ashore have been added to soil to form raised beds for potatoes and other crops. Such beds hold moisture well, but there is a concern about soil salinity (harmful to earthworms and some plants) and pollutants from contaminated water, so it is best to wash the seaweed in rainwater before use. Plants that were originally shore plants, like asparagus, cabbage, and celery are more salt-tolerant.

There is an enlightening discussion on the University of British Columbia Botanical Garden forum about using seaweed in the garden. A biologist urges rinsing the seaweed at the beach to free any creatures that might be attached to it. Even desiccated seaweed higher up on the beach harbors living things that will not survive if you unwittingly transport them with the plants you are collecting. Additionally, there may be seeds and roots of other plants you might not want to introduce into your garden.

It Is worth noting that you must have a license to harvest seaweed from Washington beaches; it is not permitted everywhere, and where it is allowed there is a ten-pound wet-weight limit. There are specific guidelines on what tools to use, and how to leave behind the base of the plant so it can continue to flourish. Be mindful that seaweed is an integral part of a complex ecosystem, and you do not want to disrupt habitat and food sources when gathering plants to use as mulch. Also heed any notices posted about pollutants that may have been released in the water where you are harvesting.

All of this being said, it does not make much sense to collect seaweed for mulch unless it is 'in your own backyard,' that is to say, you live near the beach. There are more sustainable mulch options (feed a compost pile with materials already in your garden, and use that as mulch; obtain free wood chips from a local arborist) that do not come with so many environmental factors to consider.

Date 2021-01-20
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Garden Tip

Keywords: Weed control, Organic gardening, Mulching, Herbicides

A common question we get at the Elisabeth C. Miller Library is How do I kill weeds without hand digging but without using toxic chemicals? Further discussion with the gardener reveals he wants to buy a product that he can spray on the weeds, once. Organic gardeners have it easier now compared to a decade ago, with a number of less-toxic weed killers on the market, but the fact is not one of these products is a magic bullet.

  • Corn Gluten Meal (Concern's Weed Prevention Plus and Whitney Farm's Weed Whompin Mulch) is a natural product that prevents seeds from rooting once sprouted. The downside is that it doesn't work during rainy weather. Another consideration is that recent studies show it acts as fertilizer because it is rich in nitrogen, so in garden beds it may actually increase weeds. Its best use would be for weeds in lawns, according to an article in Organic Gardening, Aug/Oct 2008.
  • Potassium salts of fatty acids (Safer Superfast Weed & Grass Killer) kills the tops of all plants, but not the roots. It works best on annual weeds like chick-weed and bitter cress, but would have to be repeated a few times to kill perennial weeds with root reserves, such as dandelion.
  • Pelargonic acid herbicide (Scythe) is another type of fatty acid, similar to soap, that kills weeds by drying out the leaves. As mentioned above this product works best on annual or biennial weeds and must be reapplied a few times to kill perennials.
  • Vinegar from the kitchen doesn't kill weeds, only disfigures them. Commercial products (Burnout, Bradfield's Horticultural 20% Vinegar) work if used in hot weather, but are quite caustic and great caution must be used not to inhale the fumes or spray the skin. Natural, yes, but toxic.
What does it take to get rid of weeds? A multi-pronged approach: physically remove weeds when they are young, reapply mulch every year, shade weeds out with desirable plants, and don't let weeds go to seed.

The Northwest Center for Alternatives to Pesticides publishes excellent articles on non-toxic pest control. Two good articles on weed management are available free online: Managing Weeds in Shrub and Flower Beds and Landscape Weed Control

Date: 2007-04-03
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Garden Tip

Keywords: Leaves, Mulching

Treat fallen leaves like the resource they are. Under the dripline of a tree, let leaves lay where they fall so nature can recycle the nutrients back to the tree. Yes, the leaves may kill the grass, but tree roots don't like the competition from grass anyway. Outside of the dripline shred leaves with your lawnmower. Mixed in with grass clippings the shredded leaves will break down fairly fast and feed the lawn.

Still feel compelled to rake those leaves? Fill a few black plastic garbage bags, add a shovel-full of soil and then stash the bags for about 9 months. You'll be rewarded with what the British refer to as "leaf mould." Use leaf mould as mulch or as an earth-friendly substitute for peat moss. Stash the bags under the deck or porch or even under the shrubbery. Just mark your calendar for next July so you don't forget. If individual leaves can still be recognized wait a few more months or use it as a mulch around perennials. For a good article on leaf mould go to the Irish Peatland Conservation Council

Date: 2007-04-03
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Garden Tip

Keywords: Leaves, Mulching, Lawns--Care and maintenance

Research from Purdue and Cornell University shows that autumn leaves can simply be left where they fall, shredded by a mower and allowed to mulch the lawn. Fertilize as you normally would. The shredding is essential, so don't skip that step. If the leaf mulch is too thick, move some into your flowerbeds or compost bin. Read the research report.

Date: 2007-04-03
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Garden Tip

Keywords: Reviews, Mulching

MULCH IT! By Stu Campbell. Pownal, VT: Storey Books. 2001
Mulch: what is it, why use it, what kind should be used? If you have ever wondered about these questions then read MULCH IT! The author describes the pros and cons of all the various types of mulch imaginable from bark to oyster shells and poultry litter, and how to use mulch around flowers, fruits and vegetables.

Date: 2002-06-19
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