Eucalyptus care

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.

on the use of wine corks as garden mulch

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.”

Hoya plant care

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.

resources on 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.

Meyer lemon plant dropping its tiny lemons

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.

when you can’t plant right away

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!

root pruning container plants

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.

vegetables to grow on roof gardens

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).

lilac and elm wood and allelopathy

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 (now archived) 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

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.”

more about soil-based potting mixes

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.