First Published 6/24/2004
A Mite Annoying

My plum tree looks like it’s having a bad hair day; the leaves are covered with clusters of three-centimetre long spikes, thrusting up from the upper leaf surface like long green warts. And I’m not alone; many gardeners are surprised to find these strange growths on many of their plants, which often leads to a bout of panic.

Mighty Annoying Mites

The weird growths, or “pocket galls,” on my plum are caused by a species of mite. These mites are impossible to see with the naked eye, and through a microscope they look like tiny, white, sausage-shaped critters, bearing little resemblance to garden variety mites.

The easiest way to identify which of the many species of gall mite is attacking your tree is by examining the mite-induced growths. For example, one species of mite, the Maple bladder gall mite, attacks silver maples, causing bright red galls to erupt on its leaves. The mite attacking my plum, on the other hand, causes those long, warty pocket galls, and is known only by its Latin name, Eriophyes emarginatae.

A Mite’s Life

The mites have a fairly simple, but interesting, life cycle. The mites overwinter in tree bark crevices as super-tough females, called deutogynes, that can endure severe cold or heat. As the weather warms in the spring and leaves start to unfold, the mites move onto the developing leaves and start laying eggs. The feeding of the developing mites causes a reaction in the tree: the leaf tissue starts to grow around the mites, encapsulating them. In effect, the tree seals off the mite from other leaves, preventing its spread. The mite, on the other hand, benefits too, since it now has a safe, secure location from which to feed.

Control Issues

If it weren’t for the often spectacular galls that encapsulate the mites as they feed on the leaves, no one would know that their trees were being attacked. But because the galls look so striking, the most common reaction is to run out to the garden centre for a bottle of spray. Fortunately, the damage caused by these mites is mostly cosmetic, having little effect on the trees’ health. Treatment, therefore, is most often unwarranted.

However, if you find these weird growths intolerable, you’ll probably want to pursue a control strategy. Once the galls have formed on the leaves, there is nothing that can be done for the current year; the galls will remain in place all season long. But in the fall, after all the leaves have fallen, thoroughly spray the trunk and branches of the affected tree or shrub with dormant oil; this will suffocate the overwintering mites.

Of course, if you have a 20-metre tall maple tree, spraying the entire tree with dormant oil will be very difficult, if not impossible, without professional help. Even then, it’s often impractical and expensive to spray the entire surface of really huge trees. The other drawback is that slathering the tree in dormant oil doesn’t just kill off the mites – it can also destroy other, more beneficial insects living on the tree, including some that actually feed on the gall mites. Unfortunately, we still haven’t discovered the perfect pest control.

Misdiagnosis is one of the biggest problems with pest control in general, and mite gall is no different. Since there are many species of mites, which attack a wide range of trees and shrubs and cause a wide range of growth distortions, the damage on the leaves is often attributed to disease. I can’t begin to count the number of times I’ve heard of gardeners spraying fungicide on ornamental cranberries and other shrubs, trying in vain to control what they thought was powdery mildew, when it was mites all along. Fungicides have no effect on gall mites, so these treatments are just a waste of time, effort, and money.

Like it or Lump it

During one of my entomology classes at university, the professor nonchalantly announced that roughly half of the students in the classroom had microscopic follicle mites residing in their nose hairs. I remember how almost everyone grabbed their noses in disgust, but the professor allayed our fears (if not our instinctive revulsion) by informing us that the mites were normal, harmless fauna, just another one of the myriad organisms inhabiting the human body.

The same can be said of many insect pests, including the leaf gall mites. While we may find the growths unattractive, the damage they cause is pretty minor, all things considered. You may attempt to control the mites or simply choose to ignore them: it all depends on whether or not you have the itch.

First Published 6/17/2004
 
Tough, fragrant, colourful, dependable: few shrubs can match lilacs. I remember leaving lilacs in nursery pots outside one winter, exposed to the cold and wind with no snow cover, and when spring arrived they remained completely unscathed. Like many gardeners, I’m a big fan of this plant.

However, because lilacs are so cherished, many gardeners feel a bit of anxiety at the thought of pruning them. Some fear that pruning will cause irreparable harm to the plant or, at the very least, stop flower formation. These worries, coupled with the prospect of an afternoon of strenuous chopping and hacking, often causes gardeners to turn a blind eye to pruning. But that’s a mistake: pruning lilacs is important to their long-term health and attractiveness. Many old, unpruned lilacs do indeed produce respectable growth with a fair number of flowers, but eventually flower numbers and quality decreases, and the branches lose vigour, becoming overgrown and untidy.

Pruning Pointers
So how should a lilac be pruned? First, you need to visualize the form that you want, and there are plenty of possibilities to consider. I’ve seen a few excellent hedges composed entirely of the common lilac, pruned to heights of about a metre and a half. And although they had attractive, dense growth habits, these lilacs sported few flowers, since they were cut back so sharply. Dense foliage and a uniform shape took precedence over blooms and fragrance. If you find the idea of a lilac hedge appealing, consider trying the varieties ‘Miss Kim’ and ‘Dwarf Korean’, which provide flowers and yet still remain thick with foliage. These varieties tend to be fairly short, reaching heights of around a meter. The relatively new “mini-lilacs,” such as ‘Shantelle’, ‘Charisma’, and ‘Prairie Petite’ are also excellent for this purpose.

Most people, I think, tend to prefer the familiar spherical, specimen-type lilac, eight to ten feet tall, rather stout, with a large number of full, fragrant flowers. If this is your idea of the perfect lilac, it’s a good idea to pick up a pair of long-handled loppers and get into the habit of annual pruning.

The biggest pruning worry that I hear from gardeners is the prospect of a big, bushy green shrub with no flowers. But you can avoid a blossom-free spring by understanding a bit about the plant’s life cycle.

Lilacs are spring-flowering plants. After the blooms of spring have finished, new flower buds form, mature during the summer, and then sit dormant through the winter. When the warm weather returns the following spring, the new buds turn to blooms by late May or early June.

Once lilacs have bloomed, the spent blooms can look a bit unsightly, and they can be pruned off. But don’t cut below the spent flowers too far, or you may cut off the buds that will become next year’s blooms.

Making the Cuts
An excellent-quality shrub-form lilac should have about seven to twelve vigorous stems, each about two to five centimetres wide; stems of this thickness are more vigorous than larger stems and produce more flowers. If you have dozens of stems emerging from the crown, you’re pretty much forced to cut off a few flower-bearing branches. But don’t fret too much over the loss – the remaining stems will provide more than enough flowers for an excellent show.  

You also have the option of creating a tree-form lilac by chopping off some of the lower branches, leaving behind a bare trunk. However, keep in mind that you need lots of smaller, secondary, 2-5 cm branches, which radiate from the larger branches, to enjoy good flower production.

Whichever form you prefer, always leave a few young branches unpruned; these will eventually replace the old unproductive wood removed during pruning. And don’t prune away more than one quarter of the shrub per year.

Finally, a word on timing: early spring is the best time to do the bulk of your lilac pruning if your goal is to form and shape for aesthetics. However, a little summer pruning is fine if you’re just removing the odd branch or two.

A Cut Above
When it comes to pruning, taking no action is an action in itself. When you choose not to prune, you’re shaping – literally – the future growth of your lilac. If you don’t mind a shrub that’s a little loose and wild-looking, with some dead twigs, this is a fine course to follow. But if you like a cleaner, tighter, more vigorous shrub, then you’ve got to take your pruners in hand and start chopping.

First Published 6/10/2004
Weed Perception

Beauty…or beast? A plant can be either, depending, in large degree, on its level of aggression. Some of the garden’s most beautiful plants can take on a beastly visage if they are given the opportunity.

Belligerent Bluebells

Take the pretty garden bluebell, Campanula rapunculoides, for example. It produces tall, attractive spikes loaded with five-pointed, violet, bell-shaped blooms, and at one time enjoyed wide popularity in gardens. However, this meek-looking plant has aggressive, fleshy rhizomes that spread rapidly through the soil, and robust, fleshy tubers that make the plant very difficult to eradicate. It takes little encouragement for this garden beauty to spread rapidly through gardens and lawns, and it will easily “jump the fence” to invade neighbouring yards. Not surprisingly, when gardeners see the plant growing out of control, they often change its name, trading the innocent “garden bluebell” for the more sinister “creeping bellflower.”

Rambling Rose

Even the venerable wild rose, Rosa acicularis, the official flower of Alberta, has a dark side. When the wild rose is nestled safely in the garden, right where we planted it, we’re happy to call it the wild Alberta rose. But when the plant succumbs to its rambling nature and spreads to places it’s not wanted, we lose our appreciation and call it the “prickly rose” instead. In fact, Alberta’s wild rose can be found not only on licence plates, but also listed as a weed in the book Weeds of the Prairies.

A Garden Full of Changelings

The bluebell and the wild rose aren’t the only changelings in the garden, of course; there are many others. And when our gardens get a little out of control, at least part of the fault is our own: we often choose plants that are tough and aggressive, hoping that these traits will help them survive our harsh winters.

It’s often the degree of aggressiveness that can transform a beautiful ornamental into a noxious weed. The very pretty purple loosestrife, with its incredible purple flower spikes, became a victim of its own success: it was so aggressive and adaptable that it soon spread from home gardens and into vast tracts of wetlands right across the country. Very few people wanted to call this plant a weed, but when loosestrife started to upset the ecological balance of marshes right across the country, ecologists had little choice but to rein it in.

There are other aggressive but desirable plants, and many of them can be safely grown in the yard if you keep an eye on them. Lily of the Valley is a great shade groundcover, but will spread like crazy given a moist, shady, uncontrolled environment. And even the innocent-looking bulb Anemone sylvestris can be surprisingly aggressive if you just let it grow as it pleases. (Fortunately, this anemone is very easy to remove and control if it tries to escape its allocated space.)

Sometimes a plant will surprise you with its aggressiveness. We once planted the very pretty Euphorbia cyparissias ‘Fen’s Ruby’ in our show garden and were shocked to see just how widely it spread, even in very dry, lean soil. It seemed to pop up at will almost randomly, several feet from its original planting spot.

Control Issues

Thankfully, many attractive but aggressive plants can be grown in our gardens, provided we take the time and effort to contain their enthusiasm. Isolation is the simplest approach; restraining aggressive plants with physical barriers such as retaining walls or planting in containers can effectively imprison spreaders. As for plants like the garden bluebell that invade lawns, it takes a lot of elbow grease (i.e., manual weeding) or an aggressive herbicide program to rein them in.

If you’re the type of person who’d rather not be bothered with containing aggressive plants, do yourself and your neighbours a favour and choose more passive species. Aggression isn’t necessarily a bad trait in plants, but the gardener who chooses invasive plants has to be willing to channel that aggressiveness in the right direction.

First Published 6/3/2004
Marigolds

Back in 1964, the marigold was chosen as the official flower of the city of Edmonton. The flower was selected because its orange and yellow colours symbolize both Prairie sunshine and the city’s role in the Klondike gold rush of the 1990s.

The people of Edmonton made a good choice: marigolds quite enjoy Edmonton’s warm, sunny days and cool nights, and the flowers are remarkably easy to grow and trouble free.

A Myriad of Marigolds

There are over thirty species of marigolds, with the African (Tagetes erecta) and French (Tagetes patula) most commonly seen in Canadian gardens. Contrary to their nomenclature, both African and French marigolds are indigenous to the Americas, native from New Mexico and Arizona down to Argentina. (When Spanish conquerors took the marigolds back to the Old World in the 16th century, the plants became well-established in France and North Africa; hence the misleading names. Interestingly, one American breeder is lobbying to change the common name “African marigolds” to “American marigolds.”)

African marigolds are the taller of the two species, growing 60 cm or taller; French marigolds average around half that height. As for bloom size, French marigolds range from 2-5 cm in diameter, while the larger African species can be 10 cm or larger. Whether planted together or separately, the two varieties put on a fantastic show in beds, borders, or containers.

The Science of Marigolds

Marigolds are not heavy feeders, but do well when given light feedings of fertilizer: a half-strength rate of 20-20-20 once a week. However, avoid adding any fertilizers rich in iron or manganese. Marigolds are highly adept at “scrubbing” these two trace elements out of the soil; in fact, they are so good at it that they can accumulate toxic doses in their leaves, which will turn the foliage a patchy brown colour.

Marigolds contain plant pigments called xanthophylls, which give marigolds their characteristic yellow colour. These xanthophylls have been used in chicken feed to darken egg yolks, wherever marketing gurus have determined that folks prefer their yolks dark yellow rather than pale. Also, lutein, a type of xanthophyll, acts as a light filter, and is purported to protect the cone cells in our eyes. But if you’re thinking of munching on African or French marigolds to protect your eyes, forget it – they taste awful.

Pest Patrol

Some gardeners claim that marigolds repel insect pests; they’re often used in companion planting for this reason. However, while it’s true that marigolds are by and large bug-free, I have seen them attacked by spider mites, aphids and thrips in the greenhouse. Marigolds aren’t the preferred food of many insects, but you shouldn’t expect them to keep all the bad bugs out of your garden.

On the other hand, marigolds have been proven to repel another class of plant pests: nematodes. Nematodes are tiny, eel-like worms that attack plant roots. While nematodes don’t inhabit the northern Prairies, they are a problem in some southern regions of Canada and the United States.

A Centennial Planting

Celebrating an anniversary with flowers is a tradition as old as human civilization. As Edmonton marks its 100th birthday, I can’t think of a better way to celebrate than to set aside at least one corner of the garden for marigolds and create a little “gold rush” of your own.