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Plant Answer Line Question

Keywords: Continuing education, Container gardening

I am going to create container plantings for some customers, and I wonder if you know the best source (book) for combining plants in containers. Also, where might I take a class in container planting?


The Miller Library has a large number of books in its online catalog on container planting, but here are a few which may be helpful to you:

Container Gardens by Number by Bob Purnell (Reader's Digest, 2004)
Contain Yourself by Kerstin Ouellet (Ball Publishing, 2003)
The Complete Book of Container Gardening edited by Alan Toogood (Quarto Publishing, 1991)
The Book of Container Gardening by Malcolm Hillier (Simon & Schuster, 1991)
A Practical Step-by-Step Guide to Complete Container Gardening compiled by Ideas into Print (Whitecap Books, 1997)

This booklist will give you an idea of the selection of books on the topic.

UW Botanic Gardens occasionally offers classes on container planting, as does Seattle Tilth.

Also, some local nurseries occasionally offer classes. One example is Swanson's Nursery.

Date 2019-09-11
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Plant Answer Line Question

Keywords: Roots, Planting, Container gardening

For how long can purchased plants remain out of the ground?


If the plants are in pots, they can stay out of the ground as long as needed. Keep them watered and they will be fine. But if they are bare root, then you should plant them temporarily (called heeling in) in a trench until you can get them into their proper holes. The most important thing to remember is to keep the roots moist. Keeping the plants out of the sun can help reduce stress as well. If digging a trench is impractical, then cover the roots with damp towels or burlap bags. Of course, planting sooner is better!

Date 2019-09-11
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Plant Answer Line Question

Keywords: Plant nutrients, Citrus limon, Water requirements, Fruit--Care and maintenance, Container gardening

I purchased a small Meyer lemon plant from a nursery in Florence, OR, and it grew, and blossomed very well, and even produced many tiny lemons - all of which have now, at this point, dropped off. The leaves are yellowing, too. It is in a good size container, in full sun. The container sits in a large saucer which does fill with rainwater. This I empty, but the plant remains wet. New blooms are coming on some of the branches, old blooms are shrivelling. No more lemons coming as yet.

My question is, why did the tiny lemons drop off? And, should the plant get overly wet? My nursery person has no information. I would appreciate any information you have.


The following information comes from Citrus (by Lance Walheim, Ironwood Press, 1996).

It sounds as if your container has good drainage, but maybe the plant is getting too much rainwater. That might be causing the leaves to turn yellow. Another cause could be a nitrogen deficiency, which would be most visible in older leaves, which would yellow from the tip to the base.

As far as the plant's water needs, it will need water when the top two to three inches of soil become dry. Frequent watering (or excess rainwater) can leach nutrients from the soil, so the plant will need to be fertilized regularly -- once or twice a month using a liquid, high-nitrogen fertilizer that includes the micronutrients zinc, iron, and manganese.

The small lemons which drop off may not be anything to worry about, as fruit drop occurs normally as the tree varies its fruit load with its carrying capacity. Pea-sized fruit usually drop about one month after bloom. A more noticeable drop occurs in late spring to early summer, when golfball-sized fruit may drop. Other reasons for fruit drop could be conditions which limit tree growth, such as excess heat, lack of soil moisture (not relevant in your case), and fluctuating weather conditions. It is also possible that the fruit drop is due to lack of nitrogen.

Date 2019-09-11
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Plant Answer Line Question

Keywords: Hoya bella, Tropical plants, House plants, Container gardening

My Hoya bella was recently moved outside. It flowered nicely, but now the leaves are a light yellow/green and the soil surface in the pot is covered with moss. What is wrong and what can I do?


Here is some information I found in the book, Subtropical plants: a practical gardening guide (by Jacqueline Sparrow and Gil Hanly, 2002, p. 107), quoted below:

Hoyas do very well in pots. They need bright light, but not sun...Hoyas strike fairly easily from cuttings, taken at the warmest time of the year.

About the yellowing of the leaves...I am pretty confident that this is due to the plant getting too much water (rain, whatever source, while it was outdoors) and the soil not drying out, which also explains what happened to the top of the soil--the moss or algae growth there. I would just gently scrape off the soil surface and put a thin layer of potting soil over it. If the plant starts getting what it needs again (as it did before it was put outdoors), it will hopefully return to its former healthy self.

During its growing season, Hoya bella prefers temps between 64 and 68 degrees; during its rest season, 59 degrees is the recommended minimum temperature (so here in Seattle, right next to a window may be too cold).

University of Florida provides additional information about Hoya bella.

Date 2019-09-11
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Plant Answer Line Question

Keywords: Container gardening, Bamboo

I am new to the area, and am renting a house that has 3 containers of bamboo plants on the deck. Two of them appear to be dead or dying, although there is still green in the canes. I tried watering them a lot for a week or so, and for one day they seemed to like that, but then they did not any more, and looked worse. Some theories people have offered: the soil is depleted, they need to be thinned, they have been poisoned somehow. Any advice? Or should I just get new ones? And, where would I get new ones?


Bamboo can grow well in containers, but it can also be picky about drainage, fertilizer and container depth.

Here is an American Bamboo Society article entitled Planting and Caring for Bamboo.

Your bamboo may have a pest or an infestation of some kind. To be sure, you may want to bag a sample of the leaves and take them to a Cooperative Extension Master Gardener Plant Clinic. Master Gardeners are trained in the identification of plants and pests and a host of other botanical subjects. To find out where to purchase bamboo locally, try Bamboo Web's sources search tool.

Date 2019-10-30
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Plant Answer Line Question

Keywords: Australian plants, Pruning trees, Eucalyptus, Container gardening

I recently purchased two Eucalyptus gunnii trees and one E. dalrympleana, which are still in their pots. I have them in full sun, facing south. I have been watering them every day - is this appropriate? I know that the gunnii tolerates waterlogged soil.


All Eucalyptus prefer full sun and well-drained soil. They are very drought tolerant when established.
Source: Manual of Woody Landscape Plants, by M. Dirr, 1998, p. 352.

If your plants are in terracotta containers they will need daily water. If they are in non-porous containers you have a bit more leeway, but do not let them dry out while they are young.

Another consideration is whether you plan to grow these trees in containers permanently, or if you are going to be moving them into the garden. If you plan to keep them in pots, bear in mind that these trees will get quite large (70 feet tall by 20 or more feet wide), so you may end up needing to do a lot of pruning from the top as well as root pruning. Sometimes, even when planted out into the garden, urban gardeners with small lots will coppice a tree like Eucalyptus gunnii or E. dalrympleana annually so that it does not overgrow its site, and so that the rounded, juvenile leaves are maintained. See the Royal Horticultural Society's page on eucalyptus pruning for additional details.

If your plan is to move the trees into the garden, it is best to do it when they are relatively young and small, as Eucalyptus generally dislikes root disturbance.

Date 2019-09-18
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Plant Answer Line Question

Keywords: Potting soils, Container gardening

I am doing container gardening -- rather large plantings that will incorporate small trees and shrubs -- and therefore want a potting mix that will last longer than the usual for smaller containers, and will provide some nutrients. I just read about soil-based potting mix, but there is no further info in my text. Can you describe this, and tell me if it commercially available, or do gardeners mix up their own recipe?


There are a variety of opinions about soil-based potting mix. Taylor's Guide to Container Gardening (edited by Roger Holmes, Houghton Mifflin, 1995) provides a recipe for a "real soil" mix combining equal parts garden loam, compost or peat moss, and coarse sand. The sand should be as coarse as possible, and should not be able to pass through a window screen. According to the guide, "the success of any mix using soil depends on the soil's quality." For large pots and planters, the mix should be equal parts coarse, medium, and fine materials (from Landscaping with Container Plants, by Jim Wilson, Houghton Mifflin, 1990), for example:
Coarse material: small nuggets of pine or fir bark
Medium material: pulverized pine or fir bark
Fine material: moistened sphagnum peat moss

Extension.org has information on organic potting mixes. ATTRA Sustainable Agriculture Program has additional information

Mother Earth News published an article by Barbara Pleasant in the December 2008/January 2009 issue entitled Make Your Own Potting Soil which should be helpful. The recipe includes pasteurized compost or soil.

Date 2019-06-05
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Plant Answer Line Question

Keywords: Water-absorbing polymers, Container gardening

I am doing container plantings for clients (some of them big -- the containers, not the clients), and have water concerns. Have you received any feedback on use of those "soil moist" granules that are supposed to cut down on waterings? My fear is that over time, especially with shrubs in a container, there may be some root rot.


Although there is not any conclusive information on whether use of water-absorbing polymers will contribute to root rot in planters, there are quite a few other causes for concern. Local gardener and writer Jessica Salmonson discusses the matter on her web site, Paghat's Garden.
Here is a brief excerpt:

Many of the 'superabsorbent' properties claimed by polymer manufacturers are exaggerated, and during biodegradation these polymers even reverse their effect, depriving plants of moisture. Woodchips, quality compost, or peat do the same job adequately, plus the woodchips or compost provide safe plant nutrients and a medium for beneficial microorganisms such as polymers retard.

And, inevitably, it turns out that some polymers do in fact reach the foodchain, especially the allegedly safer-to-the-environment biodegradable synthetic polymers. These are fed directly to livestock as feed supplements, are dispersed over crops in herbicides & pesticides, & are mixed into garden soils because of preposterous claims of doing away with a need ever again to water the garden.

Extension horticulturist and Washington State University Professor Linda Chalker-Scott has also written on this issue, and states that even beyond the health and environmental concerns, hydrogels do not work well in clay soils, and can decrease a plant's ability to absorb essential nutrients.

Local garden writer Ann Lovejoy writes about a non-polymer alternative in this article in the Seattle Post Intelligencer: "The newest such water holder I've tried is called Quench. This is a granular, sand-colored material that turns cloudy-translucent in water. Unlike polymers, Quench is based on a natural material (cornstarch) in a form that can absorb up to 400 times its weight in water, right up there with good compost or forest duff. Unlike the usual polymers, this stuff lets go with grace. About 95 percent of the stored water is released to plant roots in midsummer, making plants a lot less dependent on people in hot weather."

Date 2019-09-18
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Plant Answer Line Question

Keywords: Hemerocallis, Container gardening

We have the 'Stella d'Oro' daylilies, the Hyper Yellow variety. They are 9 years old now, going on 10. We divided them last year, and only had a couple of lilies. Where they are planted I don't believe they have enough sun. When we divided them, instead of 2 plants, we now have 7. I have a huge planter, on a wheelbase, and was thinking of putting 3 of them in it, perhaps 4. They would be located on the sunny part of our deck, which I know would be much better for them. What can you tell me about daylilies planted in containers? The pot is so big, you could grow a tree in it!


Planting your daylilies in a large container should be fine. Here is some general information from University of Minnesota Extension on growing daylilies, in which they suggest growing repeat-blooming daylilies in your containers. In general, smaller cultivars would be good choices for containers.

If you keep them in the pot over the winter, you may want to protect them by covering with a thick mulch.

You may wish to consult the frequently asked questions page on the American Hemerocallis Society website, which includes cultural information, such as the best place to plant daylilies, excerpted here:

"You need to consider four things in determining where to plant your daylilies:

Sun or Shade

  • Most daylilies do best in full sun. They will tolerate part-shade conditions, but require a minimum of six hours of direct sun per day.
  • Light yellow cultivars, many shades of pink, and delicate pastels need full sun to bring out their lovely colorings.
  • Many red and purple cultivars benefit from partial shade in the hottest part of the day because dark colors absorb heat and do not withstand the sun as well as lighter colors.

Type of Soil

  • Any good garden soil is appropriate for growing daylilies. Daylilies will grow, however, in sandy soil or in heavy clay.
  • If you have heavy clay soil, add compost, humus, peat moss, and sand to make it more friable.
  • If you have sandy soil, add compost, humus, and peat moss to lesson its porosity and to increase water retention.


  • For maximum performance, daylilies should be planted in well-drained soil. One method of achieving adequate drainage in problem areas is to prepare raised beds, 3 to 6 inches above ground level."

Date 2019-10-02
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Plant Answer Line Question

Keywords: Zinnia, Daisies, Verbena, Coreopsis, Artemisia, Salvia, Lavandula, Achillea, Echinacea, Annuals and biennials, Xeriscaping, Tagetes, Sedum, Herbs, Container gardening

Our neighborhood has a small planter area at its entrance. There is no water supply to this area, but a nearby resident is willing to water occasionally. The soil contains much clay. We would like to plant a few drought-tolerant annuals to add color and supplement the more permanent shrubs--such as boxwood--planted in the area. Can you recommend some plant choices? How could we amend the soil to best hold water during the upcoming dry months? Would a commercial product such as "Quench" be of any value, in addition to organic mulches?


I found the following article by Nikki Phipps on GardeningKnowHow.com about drought-tolerant container planting. Here is an excerpt:

"...many plants not only thrive in containers but will tolerate hot, dry conditions as well. Some of these include annuals like marigolds, zinnias, salvia, verbenas, and a variety of daisies. Numerous perennials can be used in a xeriscape container garden such as Artemisia, sedum, lavender, coreopsis, Shasta daisy, liatris, yarrow, coneflower and more. There is even room for herbs and vegetables in the xeriscape container garden. Try growing oregano, sage, rosemary, and thyme. Vegetables actually do quite well in containers, especially the dwarf or bush varieties. There are also numerous ornamental grasses and succulents that perform nicely in containers as well."

This Brooklyn Botanic Garden's 2001 article provides a list of drought-tolerant plants for containers.

I had not heard of Quench, but since it is cornstarch-based, it is certainly preferable to the hydrogel and polymer products which are more widely available. I found an article by garden writer Ann Lovejoy in the Seattle P-I (June 3, 2006) about Quench. Here is an excerpt:

With pots and containers, mix dry Quench into the top 12 inches of potting soil in each pot and top off with plain compost. Few roots will penetrate deeper than a foot, so it isn't very useful down in the depths of really big pots unless you are combining shrubs and perennials.

I would not recommend hydrogels or polymers as a soil amendment. Professor Linda Chalker-Scott of Washington State University has written about these products and their potential hazards. Here is a link to a PDF.

You could consider applying a liquid fertilizer (diluted seaweed-fish emulsion would work) to your containers once every week or two during summer. Here is an excerpt on some general information on container maintenance, from a no longer available Ohio State University Extension article. Excerpt:

"Once planted, watering will be your most frequent maintenance chore, especially if you are growing plants in clay containers. On hot, sunny days small containers may need watering twice. Water completely so that water drains through the drainage hole and runs off. Water early in the day.

"If you incorporated a slow release fertilizer into the potting mix, you may not need to fertilize the rest of the season; some of these fertilizers last up to nine months. You can also use a water-soluble fertilizer and apply it according to the label directions during the season.

"Mulch can be applied over the container mix to conserve moisture and moderate summer temperatures. Apply about one inch deep.

"Depending on the plants you are growing, you will need to deadhead and prune as needed through the season. Monitor frequently for pests such as spider mites. Pests usually build up rapidly in containers."

Date 2019-09-19
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Plant Answer Line Question

Keywords: Magnolia, Container gardening

I recently purchased a magnolia that had no tags on it. I have an extremely large pot that I would like to plant the magnolia in. My books at home lead me to believe that I should plant it in Azalea and Camellia potting mix. A local nursery has advised me that this would be fine, although another has said no. They also disagreed with my plan of putting rocks, bitumen, and old leaves in the bottom of the pot to help with drainage. They believe a quality potting mix and nothing else is the way to go. What are your suggestions?


Here is what the book, Magnolias: A Gardener's Guide, by Jim Gardiner (Timber Press, 2000) says about growing Magnolias in containers:

...considerable experience is needed to retain magnolias in a container for any length of time. The roots are particularly sensitive to being hot and dry during the summer months and frosted during the winter months... Evergreen magnolias and clones of Magnolia grandiflora, in particular M. grandiflora 'Gallissonniere,' can be grown in very large containers for indoor use in atria.

I think if you take the matter of extreme heat and cold into consideration, you should be able to grow your magnolia in a container. I would be curious to know which species you have, because some get very large, and for these a container might not be a good choice. Magnolias prefer good, free-draining acidic soil that does not dry out, according to Rosemary Bennett's book, Magnolias (Firefly Books, 2002). Since Azaleas also prefer acidic soil, the idea of using Azalea and Camellia potting mix makes sense.

You may find the following information on growing trees in containers helpful:

Virginia Cooperative Extension: Trees for Landscape Containers and Planters

University of Tennessee Extension: Trees to Plant in Containers or Wells

UBC Botanical Garden Forum: A discussion on requirements for magnolias in containers

UBC Botanical Garden Forum: A discussion on potting guidelines for a particular magnolia This discussion suggests that the container should be filled with soil-based compost which provides some nutrients to the plant.

As for container drainage, here is what Prof. Linda Chalker-Scott of Washington State University says. In short, she says that putting coarse material in the base of a pot for better drainage is a myth.

Date 2020-01-23
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Plant Answer Line Question

Keywords: Vegetable gardening, Container gardening

My question is about the ceramic pots that you see in nurseries and places in the area. The pots are glazed on the outside, and unglazed on the inside, and they are made in China and Vietnam. Are these pots safe for planting vegetables and herbs? Or, are there materials in the interiors of the pots that could leach into the soil and make the vegetables and herbs unsafe to eat?


Some ceramic glazes do contain toxic materials, such as lead and cadmium. Washington State Department of Health has information on preventing lead poisoning, and on testing for lead.

State of Oregon's Lead Poisoning Prevention Program includes information on sources of lead exposure, including pottery.

California Department of Health has several pages on toxins in pottery.


"The U.S. Food and Drug Administration (FDA) sets standards at the national level for the amount of lead that can pass out of, or 'leach,' from dishes. Tableware with lead levels greater than these standards cannot legally be sold in the U.S. The FDA regulations cover only tableware that is imported or that is brought into the state for sale. The standards apply only to items that are used for foods and beverages. They do not apply to pieces that either cannot hold liquids or are not intended to hold liquids, such as salt shakers, cookie jars, butter dishes, etc. See the table below for the FDA standards for lead in ceramicware.

Decorative ceramics
The FDA has labeling rules for ornamental or decorative ceramics that are not intended for food use. These items must either (1) be permanently labeled with a logo or statement that they are unsuitable for food use, or (2) be made incapable of holding liquid. If an item is clearly intended for food use, such as a bean pot, labeling it is not sufficient, however. It must be made unusable, for example, by having a hole drilled through any surface that could hold liquid."

My co-worker tells me that some retail stores are good about informing customers if pots are unsafe for food use. This document from Clemson University Extension (although its focus is cookware) suggests that you not use pottery which does not bear the label, "Safe for Food Use:"

If a pot has been fired at a high temperature (something you cannot easily ascertain by looking at it), my thought would be that there would be less likelihood of toxic material from the glaze leaching inward, but if the clay itself comes from a source which is full of contaminants, there may be a risk apart from the glaze. If you are at all concerned about using these pots for growing food, my advice would be not to do it. There are other ways of growing food in containers, such as untreated wood boxes or barrels. See links here for general information on growing vegetables in containers:

Vegetable Gardening in Containers from Virginia Cooperative Extension.
Plants Grown in Containers from North Carolina State University.

Date 2020-01-18
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Plant Answer Line Question

Keywords: Quirky, Potting soils, Herbs, Container gardening

I have a huge planter to fill but don't want to buy that much soil so I want to partially fill it with wood. I'm going to plant herbs in it but I wanted to know if the wood I have would make eating the herbs inadvisable. I have roots and branches from a snake bark elm and some large pieces of lilac. None of the wood is treated but I know some wood is poisonous and wasn't sure about these two.


Before you go ahead with using wood to fill in the planter, another trick you might try is to put an upended smaller pot inside the large pot, if the planter is too deep. What you are looking for is a potting medium with good drainage.

I am not familiar with snakebark elm (there is a snakebark maple, and a lacebark elm--might it be one of these?) so I can't give a conclusive answer about its wood or roots. The phenomenon of plants which are toxic to other plants is called allelopathy. The most famously allelopathic tree is the black walnut. Apparently, lilac wood (Syringa vulgaris) has the ability to raise the phenolics content in the soil, according to a 2004 scientific article I found, from the 2nd European Allelopathy Symposium.

To be on the safe side, I would avoid using the lilac and elm wood as filler in your planter, since there are better options.

You may find the information below useful:

Local gardener Mary Preus's book, The Northwest Herb Lover's Handbook (Sasquatch Books, 2000) offers a recipe for potting soil for herbs grown in containers:

  • 8 quarts compost, earthworm castings and/or composted chicken or steer manure
  • 4 quarts sphagnum peat moss
  • 4 quarts perlite
  • 4 quarts builder's sand
  • 1 cup all-purpose fertilizer mix (she has another recipe for this*)
  • 3 tablespoons ground dolomitic limestone

    *all-purpose fertilizer recipe:
  • 2 pounds fish meal or crab meal
  • 1/2 pound greensand
  • 1/2 pound steamed bonemeal
  • 1 pound rock phosphate
  • 1 pound kelp meal

Virginia CooperativeExtension also has information on soil mixes for growing edible crops in containers:

"A fairly lightweight mix is needed for container gardening. Soil straight from the garden usually cannot be used in a container because it is too heavy, unless your garden has sandy loam or sandy soil. Clay soil consists of extremely small (microscopic) particles. In a container, the bad qualities of clay are exaggerated. It holds too much moisture when wet, resulting in too little air for the roots. Also, it pulls away from the sides of the pot when dry.

"Container medium must be porous in order to support plants, because roots require both air and water. Packaged potting soil available at local garden centers is relatively lightweight and may make a good container medium.

"For a large container garden, the expense of prepackaged or soil- less mixes may be quite high. Try mixing your own with one part peat moss, one part garden loam, and one part clean coarse (builder's) sand, and a slow-release fertilizer (14-14-14) added according to container size. Lime may also be needed to bring the pH to around 6.5. In any case, a soil test is helpful in determining nutrient and pH needs, just as in a large garden."

Date 2019-08-02
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Plant Answer Line Question

Keywords: soil contamination, Container gardening

Is it safe to use galvanized containers (in this case a large galvanized tub like the kind used for watering livestock) in which to grow root vegetables, herbs, etc? I've seen some sites sell these for this use, but there are also some postings referring to the potential for ill effects of anti-rust coatings.


You may want to consider both what goes into the galvanizing process, and what could be leaching out of containers as the coating wears down over time.

Here is information on the process of galvanizing from the American Galvanizers Association. Excerpt:

"What are the steps in the galvanizing process?"

  1. Pre-inspection - where the fabricated structural steel is viewed to ensure it has, if necessary, the proper venting and draining holes, bracing, and overall design characteristics necessary to yield a quality galvanized coating
  2. Cleaning - steel is immersed in a caustic solution to remove organic material such as grease and dirt, followed by dipping in an acid bath (hydrochloric or sulfuric) to remove mill scale and rust, and finally lowered into a bath of flux that promotes zinc & steel reaction and retards further oxidation of the steel... (steel will not react with zinc unless it is perfectly clean).
  3. Galvanizing - the clean steel is lowered into a kettle containing 850 F molten zinc where the steel and zinc metallurgically react to form three zinc-iron intermetallic layers and one pure zinc layer

Based on the above, one concern would be whether the zinc would be harmful. Zinc is one of many nutrients needed by plants, but I couldn't hazard a guess as to what effect the zinc from the coated steel would have, if any, or whether the galvanizing process involves other substances.

The book The Edible Container Garden by Michael Guerra (Fireside, 2000) says the following:

"Galvanized buckets are increasingly popular but don't use them for ericaceous or acid composts." (This would be a compost which is lime-free. Usually soils in the Pacific Northwest tend to be acidic. I don't know from your message what part of the country you live in, but this might be something to consider as well).

Another issue is that the metal containers will probably heat up quickly, meaning that your plants might need more attentive watering.

Los Angeles County Cooperative Extension offers the following information, in an article on trace elements and urban gardens. Excerpt:

"Cadmium is a contaminant of many manufactured products containing zinc. Any zinc plating or galvanizing operations and galvanized metal containers sometimes used in horticulture and gardening operations are potential sources of cadmium."

I certainly don't think you want to grow vegetables in a cadmium-laced container. In situations where there is any doubt about safety, I would recommend growing ornamental plants in the tubs, and growing edibles in untreated wood or clay pots.

Date 2020-03-27
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Plant Answer Line Question

Keywords: Green roofs (Gardening), Vegetable gardening, Container gardening

I'd like to plant a vegetable garden on my roof. It will be in a feed trough about 8 feet long, and 2 feet wide and deep. I'm wondering what I can add to lessen the weight of the container (so it won't just be filled with potting soil and compost). Also, any recommendations for which vegetables to grow would be great--things which are fairly easy and don't have enormous roots!


To lighten the load of your container, a lightweight organic material like hazelnut shells might make a good bottom layer. You could use perlite, but that may actually be heavier than the nut shells. Here is information about sources of hazelnut shells:
Oregon Hazelnuts (a website of hazelnut growers) (lists several sources)
A Washington State source, often found at local farmers' markets, is Holmquist Hazelnuts.

The book The Edible Container Garden: Growing Fresh Food in Small Spaces by Michael Guerra (Simon & Schuster, 2000) has a section on rooftop containers, and recommends (after you've consulted a structural engineer) using lightweight, well-draining compost, and setting your container(s) on timbers to help with drainage. According to the book, the best candidates for containers are potatoes, chard, lettuce, radishes, shallots, bush tomatoes, eggplant, zucchini, squash, dwarf carrots, dwarf beets, mustard and Asian greens, and runner beans. More difficult are cauliflower, cabbage, Brussels sprouts, kale, parsnips and other deep-rooted vegetables.

The following links may be of interest:
University of Maryland Cooperative Extension
Vegetated Roofs from Oregon State University Extension
Article about the Reading International Roof Garden (Britain) from The Guardian by Emma Cooper (and another article by this author in Permaculture Magazine #53).

Date 2019-08-07
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Plant Answer Line Question

Keywords: root pruning, Container gardening

I'm interested in information about root pruning container plants. When and how? Do the plants have to be removed from the containers in order to do it? Do the cut roots need to be removed from the container too?


You will need to remove the container in order to prune the roots. Don't leave cut roots in the container.

Ann Wheeler of Log House Herbs demonstrates how her root pruning technique in this Arbor Gate video.

According to Sunset's Container Gardening (1998), fall is the best time to do this (when the plant is not in the height of the growing season). They suggest that a containerized plant can be root-pruned after 3 years or so. Here are the steps they recommend:

  • Loosen the soil around the pot walls with a knife.
  • Tap around the rim with a rubber mallet and then pull the plant free - OR:
  • float the root ball out by forcing water from a hose through the drainage hole
  • Examine the root ball and trim any twisted roots.
  • Pull out and untangle large roots, and then use shears or a pruning saw to cut big roots back by 1/3 to 1/2.
  • Scrub the inside of the pot with a stiff brush and plain hot water or a solution of 4 parts water to 1 part bleach. Rinse with clear water.
  • Repot with fresh potting mix.

Date 2019-08-07
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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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Garden Tip

Keywords: Irrigation, Container gardening

Research shows that watering container plants in the afternoon leads to healthier, stronger growing plants compared to containerized plants watered early in the morning. Warren and Bilderback, Journal of Environmental Horticulture, September 2002, Vol. 20(3), Pages 184-188. However, watering the rest of the garden in the morning reduces the amount of water lost to evaporation, which is better for the utility bill!

Date: 2007-05-16
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Garden Tip

Keywords: Container gardening

The secret to creating a professional looking seasonal container is to leave no open space between young annual transplants. When planting annual flowers in the ground generous spacing allows plants to grow bigger. However, planting in pots is different because the limited soil means plants need supplemental water and regular feeding and therefore more plants can be supported in a smaller area. A couple plants may be smothered by larger plants after a few weeks, but by then the lush growth will be a sight to behold.

Container Gardening Complete by Jessica Walliser (Cool Springs 2017) covers the basics of growing plants in pots and also has many fun "recipes" for thematic containers such as "butterfly garden tubs" and building a "beet box" from repurposed CD storage boxes.

Date: 2018-07-19
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Container Theme Gardens   by Nancy J. Ondra, 2016

Reviewed by: Brian Thompson on 2016-05-01

container theme gardens book jacket It’s time to plan and plant your summer container gardens. But what plants will you select? "Container Theme Gardens", a new book in the Miller Library, will help you choose, with 42 different designs that will meet almost every need: sun or shade, flowers or foliage, ornamentals in your favorite color or a mini-kitchen garden that will fit on your balcony, deck, or windowsill.

Salad greens and herbs are naturals for this treatment; more unusual are plants grown for their berries or for making tea. Other selections are designed to attract: hummingbirds, butterflies, even your kids. Best of all, this is only the newest book in our excellent collection on containers that will help you plant a garden no matter how limited your space or time!

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