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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: Eucalyptus, Allelopathy

I have a young Eucalyptus pulverulenta tree. I'm concerned that it might be emitting toxic chemicals into the surrounding soil that will harm other plants. Is this indeed a cause for concern?


My instinct is that your Eucalpytus tree will probably not be toxic to most other plants. Eucalyptus is often seen growing in the midst of garden beds here in Seattle, to no ill effect. I have a tree growing in a perennial bed, and have noticed no problems. However, the leaves do contain essential oils (phenolics and terpenoids). This does make the tree more flammable, if that is a concern. Eucalyptus leaf litter (to a greater degree than exudates from the tree's roots) can inhibit specific food crops, like wheat. This is what is meant in this University of Florida Extension article, which refers to "selective activity of tree allelochemicals on crops and other plants."

To show how selective this chemical property can be, see the following documentation of a University of California, Davis experiment using composted Eucalyptus as mulch.
"Mulch products made from Eucalyptus are an asset to maintenance of fine landscapes. Composted Eucalyptus makes an excellent seed cover and will aid in germination and establishment of seedlings such as California Poppy. Fresh Eucalyptus is an excellent mulch for woody landscape plant materials and palms; the main effects of its use are weed control and water conservation."

Unless you are growing your tree in the middle of a field of wheat or among other grassy plants, your landscape is probably in no danger of inhibited growth from the Eucalyptus.

You may find the following veterinary perspective (from Cornell University) on the toxicity of Eucalyptus of peripheral interest.

Date 2019-10-02
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Plant Answer Line Question

Keywords: Insect pests--Control

Is there really a plant that will ward off mosquitoes, and if so, what is the name and is it available in the Seattle area?


There is disagreement about the extent to which certain plants repel mosquitoes. Below, please find some web sites that highlight some plants that may work.

There are eleven plants generally thought to repel mosquitoes:

Citronella, Eucalyptus, Pennyroyal, Rosemary, Rue, and Wormwood. Milder ones (in our experience) include Basil, Bay, Lavender, Sage and Thyme. With even the smallest of herb gardens, or access to a supermarket selling freshly-cut herbs, the leaves of such plants can simply be rubbed on pets and people to temporarily ward off insect attacks.
(Source: Janette Grainger & Connie Moore. Natural Insect Repellents for Pets, People & Plants. 1991, p.11.)

According to Donald Lewis of the Department of Entomology at Iowa State University, citrosa, lemon thyme or citronella grass may help repel mosquitoes, but you have to crush the leaves and rub them on your skin to make them work.

According to the MadSci Network, citronella oil may be more effective at repelling mosquitoes than the plant itself.

Lastly, Colorado State Cooperative Extension recommends scented geranium, lemon grass and a host of other plants.

There are many local nurseries which may carry the plants mentioned above, but since inventory changes frequently and they do not list their inventory online, it is best to give them a call to find out if plants you are seeking are available.

Because of the ongoing concern about West Nile Virus, there is a lot of information available on ways to control mosquitoes. See King County Public Health's resources on this topic.

Date 2019-09-27
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Plant Answer Line Question

Keywords: Dictamnus, Quirky, Firescaping

My "gas plant," Dictamnus albus, is finally flowering for the first summer ever, and I am starting to worry: can it spontaneously combust? It's planted close to the house. I remember stories from a couple of years ago about houses in Seattle catching fire because of sun or extreme heat igniting compost or soil in planters. Are there plants besides Dictamnus that are especially flammable?


Dictamnus is not called the gas plant or the burning bush for nothing. It is in the same family as citrus plants, and contains extremely volatile oils that can indeed reach a high enough temperature to ignite. In Defense of Plants blog describes this aspect of the plant, and asks why a plant might have this capability (to burn out competing vegetation, or merely an unintended consequence of oil production). Excerpt:
"If air temperatures get high enough or if someone takes a match to this plant on a hot day, the oils covering its tissues will ignite in a flash. The oils burn off so quickly that it is of no consequence to the plant. It goes on growing like nothing ever happened."
Some gardeners amuse themselves and amaze their friends by demonstrating this flare of flame, but I highly recommend you not try it if your plant is up against your house!

You can read more about the flammable properties (and garden merit) of Dictamnus in the June 1995 issue of American Horticulturist. See the article "Ignite the Night" by Robert L. Geneve.

There are other flammable plants. Areas that are accustomed to preparing for summer fire season (such as Grants Pass, Oregon) have information about which plants are most (and also which are least) likely to ignite. The flammable list includes ornamental juniper, Leyland cypress, Italian cypress, rosemary, arborvitae, eucalyptus, and some ornamental grasses.

If you are concerned about the proximity of this plant to your house, you might consider transplanting it elsewhere in fall, though be aware that Dictamnus has a taproot and is not fond of being moved.

Date 2019-07-26
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