Elisabeth C. Miller Library logo Miller Library Home UW Botanic Gardens Home UW Botanic Gardens Home book graphic

3501 NE 41st Street, Seattle, WA 98195 | (206) 543 0415 | Open: | Library Schedule

Gardening Answers Knowledgebase

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

The National Sustainable Agriculture Information Service has extensive information on organic potting mixes.

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 2017-05-05
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Insect pests--Control, Potting soils

I need advice on how to rid my house of fungus gnats which were introduced in a bag of potting soil I used when repotting my houseplants. One plant is difficult to repot because it has long branches that cascade down the side of the pot in an intertwined mass.

I always let the soil dry out completely before potting, and also in between waterings. Recently I added a bunch of sand to the top of the soil. Would repotting again help? Is there a no-pest strip that is safe for use indoors for this insect? (I'm chemically sensitive and also concerned about the soil's fungus).


I'm sorry to hear of your struggle with fungus gnats. I consulted University of California, Davis Integrated Pest Management online, and here is a link to their page on this insect and methods of controlling it. Here are excerpts which may be relevant to your situation:

Purchase and use only pasteurized container mix or treat potting soil with heat or steam before using it; this will kill flies as well as the algae and microorganisms they feed on. Store pasteurized potting soil in closed containers to prevent it from becoming infested before use.

Commercially available Steinernema nematodes, Hypoaspis mites, or the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti) can be applied to control fungus gnat larvae in container media.

North Carolina State University Extension also has suggestions on indoor control of this pest. Excerpt:

Potted plants and other types of interiorscaping are often the culprits. Check plants to see if the soil is excessively wet. Drain any excess water from the dish below the pot. If the weather permits, move the plants outdoors or allow the soil to dry down (not to the point where plants wilt). You can also drench the soil as mentioned previously. Then, increase the interval between regular watering and the problem should abate.

If you can possibly repot the plant(s) which had the infested soil, and use sterile potting soil, that should help. If this doesn't work, the Bacillus thuringiensis (Bt) or parasitic nematodes might be an option. I think the Bt might present problems for your chemical sensitivity, as you would need to avoid breathing it in, and prevent it from getting on your skin and clothing. However, the Steinernema feltiae nematodes should not be a problem at all. One example of a source for these is Peaceful Valley Farm Supply.

I don't think adding sand on top of the potting soil will be effective. It might actually create a kind of crust over the top of the soil, causing a drainage problem. If you are concerned about fungus in the soil, using sterilized or pasteurized potting soil is a good idea. You can try using yellow sticky traps to catch the gnats; it can't hurt, although it won't completely solve the problem unless you are willing to repot with new soil. Most garden centers sell these traps, or you can make your own as described by New Mexico State University Extension.

You can also employ trapping techniques using yellow sticky traps. These may be purchased, or you can make them from yellow surveyors tape or yellow plastic butter tubs, etc., coated with vegetable oil, Vaseline, or other sticky material. Put these traps in a window or other well-lighted location. The adult gnats are attracted to the yellow color and get stuck on the trap. This removes them from the home environment and reduces their ability to reproduce. (They die on the trap.) After you catch a lot of gnats, just discard the whole trap or wipe the insects off and reapply the sticky material mentioned above, and you are ready to catch more.

Date 2017-04-19
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Herbs, Quirky, Potting soils, 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

West Virginia County Extension also has guidelines for potting soil in containers. Excerpt:

"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 2017-05-05
Link to this record only (permalink)

Plant Answer Line Question

Keywords: Cold protection of plants, Grevillea, Potting soils

I have a Grevillea victoriae that I bought 2 years ago. It's about 2 ft tall and is in a pot with bagged soil and sand added for better drainage. It hasn't bloomed much, but it was trying. This year it has a lot of frost damage, and I'm not even sure it's alive.

When can I expect to see new growth? Should I prune off the damaged areas and cross my fingers?


The usual rule of thumb about winter-damaged plants is to scratch the surface of a branch or stem with your fingernail to see if there is green underneath. If there is, that is a good sign. Then, you should give the plant until late June or early July to show signs of resurgence from the damage. I will include below an entire newsletter item on this topic from Plant Amnesty's Cass Turnbull:

January 2009 e-mail newsletter: Is Your Frozen Shrub Dead? by Cass Turnbull
"After an extraordinarily cold winter in Western Washington, many garden owners will want to know what to do about the damage to many of our not-completely-hardy shrubs. With many of our broadleaf evergreens, it's common for their leaves to turn brown or black and eventually fall off. The plants themselves are probably still alive. To check, use a hand-pruner blade to peel back a little bit of the "skin" to see if the cambium layer just beneath is alive (green) and not dead (brown). If alive, it'll probably flush out with a new set of leaves. So don't panic if you shrub looks dead. Wait and see. How long? By June you will have an answer.

"By then, those that can put on a new set of leaves will have done so. If you can't stand the sight of the stricken brown shrub until June, try running your hands along the branches to knock the brown leaves off. Then, the plant might seem to be deciduous, not dead. By the end of August, the final report will be in. Freezing weather sometimes does internal damage that doesn't show up until after the stress of the summer "drought". A shrub may look okay through June and July, but then, while it is pumping H2O like crazy trying to keep up with the heat demand in August, some portions can collapse, and you will see die-back. (The non-scientific explanation is my own and may be a little, well, anthropomorphic.)

"Many evergreen shrubs, such as escallonia, that suffer freeze damage, will die from the tip back. These shrubs respond well to radical size reduction which in this case means big ugly cuts to the point of green wood. The plants will "break bud" just below your cuts and many new green-leafed shoots will rather quickly grow out to hide the cuts and provide you with a "new" plant by the end of the growing season.

"Often, (for example,in the case of choisya), branches will split, break or splay flat to the ground due to snow loading. Get your loppers out and whack everything back to 4" to 6" off the ground. Yes, it's really Okay. I promise. I have done this thing many times. As soon as the growing season begins, the majority of cut plants will spring into action. As the renovated shrubs grow up, it is advisable to pinch them back every so often, to encourage branching and thicken them up. 'Pinching' means a very light heading, just nipping the end bud of each branch with your fingernails or hand-pruners."

You may find this link to a Northwest grower of Grevilleas of interest, too.

On another topic, I'm curious about the mixture of potting soil and sand you are using for your Grevillea victoriae. If you were to add sand to most Seattle-area garden soil (which tends to be clayey), you would end up with poorly draining concrete-like soil. I would assume that potting soil which already has perlite or something similar in it for drainage and would not need an addition of sand. Are you planning to move the shrub into the garden at some point? This is a substantial shrub--the mature ones I've seen are at least 6 feet tall by the same width. If you can find a place in the garden for it, it might fare better. As you probably know, plants in pots tend to be more vulnerable to extreme cold.

Date 2017-08-15
Link to this record only (permalink)

Garden Tip

Keywords: Plant nutrients, Soil testing, Potting soils

Test your soil for Ph and nutrients before your next planting project. Our county extension service no longer tests soil. Soils and Soil Testing Information from the Miller Library has a list of places to send samples and how to collect them.

After you have your soil test analysis with its recommendations for 200 pounds of nitrogen per acre, use these handy conversion tables to convert that to your 100 square foot P-patch. Fifty other tables and formulas will help you convert just about anything you might need for the garden, including how much potting soil you will need to fill those 10" flower pots. Conversion Tables, Formulas and Suggested Guidelines for Horticultural Use from the Cooperative Extension Service, University of Georgia College of Agricultural and Environmental Sciences.

Date: 2007-02-20
Link to this record (permalink)

Didn't find an answer to your question? Ask us directly!

Browse keywords

Search Again:

August 01 2017 12:36:01