Aquatic plants as mulch?

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.

Mulching, Myriophyllum spicatum, Seaweed

Colored bark mulch

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.

Mulching, Soil contamination

Mulching around irises

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.

cherry tree leaves and cyanide

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.”

raking leaves in the Fall

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.

using rice straw as compost or mulch

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.

cedar wood chips and 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.

cottonseed hulls as mulch

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.

on the use of Casoron and Preen for weed control

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(now archived) 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.

mulching Pacific Northwest native plants

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…”