Questions - Rhodos and Lavatera

Back to the questions: what to do about greenhouse white flies; when to transplant a Lavatera shrub; and what’s troubling a rhododendron?
by Art Drysdale
by Art Drysdale


Art Drysdale, a life-long resident of Toronto and a horticulturist well known all across Canada, is now a resident of Parksville, British Columbia on Vancouver Island, just north of Nanaimo. He has reno-vated an old home and has a new garden there. His radio gardening vignettes are heard in south-western Ontario over radio station Easy 101 FM out of Tillsonburg at 2 PM weekdays.

Art also has his own website at

August 27, 2006

Two shots of damage caused y the black vine root weevil; at top, the typical foliage holes from the adults, and below, dead and dying Japanese yews due to eating of their roots by the larvae. Author photos.

Back to questions this week. Ray Johnson, of Cambridge Ontario, a listener to my AM740 radio programme writes: “I have a small hobby greenhouse, and have been having a MAJOR problem with White Fly during the winter season, specifically on the tomatoes. I have tried a number of insecticidal soap solutions which do not seem to control them.

Question - do the eggs? of these pest live in the soil in the greenhouse?

Question -if so, is there a product in which you can apply to the soil to kill them -or- is there a product that can be used to kill other pests in the soil? Obviously I do not have the capability to steam the soil in the house to kill any pests.

One of the most annoying pests both indoors (especially in greenhouses or conservatories) or outdoors (particularly on tomato and Lantana plants) is the greenhouse white fly. There are now at least three separate and distinct types of this insect, and control of them depends on which one is present. The most common and oldest is Trialeurodes vaporariorum. They produce new generations every few days, although an individual life cycle can take up to 40 days to advance from egg through four nymphal stages to pupae and finally adult. The middle three nymph stages do not move on the plant. Eggs are laid on the underside of the uppermost leaves on plants. The insect generally never makes contact with the soil.

Various insecticides can be used, including those that are industrial-soap-based. The latter, particularly, must be applied at least every three or four days. I would suggest Doktor Doom’s House & Garden Insecticide which contains Permethrin, and thus you will have a little bit of a longer-lasting effect. It comes in an aerosol container and is easy to use.

The other prime weapon gardeners have against all white flies is the sticky yellow cards. These are available from at least one manufacturer but are simply made by obtaining a large sheet of bright yellow Bristol board (or equivalent) from an art supplies store. Cut it up into cards about four by six inches, and punch a hole in the centre of the narrow width near the edge. Coat each one with any sticky substance such as molasses or even double-sided ‘Scotch tape’. Tie these onto stakes or otherwise suspend them so they are at about the height of the top of the plants affected. Growers generally use one card per 90 m2 (1,000 sq. ft.) and usually replace them weekly.

Resultant from my short gardening vignettes weekly on Shaw TV on Vancouver Island, Betty Hilke wrote with the following question: “I have a pink blossomed Lavatera which is practically at the end of blooming and of course all branches are quite leggy. I want to move it to a new location--when is the best time to do this--now or in the early spring? Also when is the best time to prune it as I have to prune it before I can move it, as it is too much for me to handle? Thank you.”

I cannot think of a single reason that you would not be able to move your Lavatera late this fall or this winter here on Vancouver Island. Obviously you should prune it hard (back 1/3 to ½ at least). We have even moved them here in mid-spring after the new leaves had emerged!

A great shrub, and easy to care for. If you just have the standard pink-flowering one, you should look for other cultivars such as ‘Barnsley’ and ‘Brendon Springs’.

Leonce and Barbara Gagnon of Surrey B.C. wrote to Donna Dawson at as follows: “My friend has two Rhododendron plants bought at the same time located about 15 feet apart. One is about 4 feet tall and doing very well. The other has been looking almost dead for 4 years. The stalks are brown and lifeless but there are a few green leaves on the ends of a number of the "dead" branches. She thought perhaps she should transplant the sad 2 foot plant. I suggested she not disturb the roots until we contact your place. What do you suggest?”

It’s difficult to diagnose a plant problem with so little information. For example, disturbing the plant; i.e. checking its root system may be a very good idea. You might find the roots loaded with larvae of black vine root weevils which kill many plants. If they are present, the adults should also be damaging the foliage, leaving half-moon-shaped ‘holes’ at the edges of most leaves, possibly also on the ‘healthy’ plant.

There could be something negative to these acid-loving plants (such as buried drywall) at the roots. Something in the soil, such as black muck additive that is difficult to wet, may be preventing the plant’s roots from absorbing sufficient moisture and nutrients.

Knowing the type of soil too would be valuable, and the sun/shade exposure of the two plants could help. The colour of the foliage on the two plants might help diagnose a lack of a specific or several nutrients.

I mention these only as examples of what may be wrong, but cannot go further without knowing more.

You might wish to make the inquiry of the garden centre at which you purchased the plant. It is always possible that it was a weak plant that has made little root growth. That too might be ascertained by digging the plant once the cooler, moist weather returns.

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