Q: When is it too late to do dormant pruning? I had planned another round of winter pruning of the veteran apple and pear trees on our property, but they are already budding before I’ve been able to get to them (seems like growth is starting early this year).
A: You can still prune now; November through late March is considered pruning season. You want to get it done before bloom. And you can prune out dead, diseased or dying wood any time of year.
— Erica Chernoh, Oregon State University Extension horticulturist
Pruning a young crape myrtle
Q: I was just told that I should prune my newly planted 6-foot crape myrtle tree. I was not aware of this. Is it too late to prune it? If yes, how much should it be trimmed?
A: Crape myrtles flower on wood produced in the current year, so any pruning should be done in early spring. It is best to prune as little as possible. Remove congested wood on the interior of the plant to maintain good air circulation, which helps avoid powdery mildew. Gently shape the remaining branches to enhance their natural character. As trees mature, removing the lower limbs will reveal the striking bark on the trunks. Pruning will encourage new growth but does little to enhance flowering.
Excessive pruning causes stunting and leads to unattractive scars that require time to heal, and the rank growth stimulated by severe pruning may make a crape myrtle more susceptible to disease and cold damage. The most common outcome is vigorous but weak growth that is unable to support the heavy trusses of flowers, and some branches may even snap at their peak of bloom.
— Chris Rusch, OSU Extension master gardener
Pruning overgrown lilac bushes
Q: I have some very overgrown lilacs. They each have a few limbs that massively overhang the yard. I understand the concept of removing old branches over a three-year period, one-third at a time. Due to the timing of an event we’re going to have in our yard, I’d like to take this year’s third off now rather than wait until after it blooms. I’d be leaving the other two-thirds of the plant alone. Will it cause any harm to the bush overall if I do this now?
A: The rule of pruning no more than one-third of a plant exists so that the plant retains enough foliage to produce “food” for the tree (through photosynthesis) while growing plant structure from nodes below the pruning cut. I would suggest that you at least wait until after the last frost if you choose to ignore another rule: to prune after blooming. If the pruning is done correctly, your lilac should suffer no long-term ill effects.
— Kris LaMar, OSU Extension master gardener
Rhododendron has drooping leaves
Q: Are drooping leaves on a rhododendron a symptom of a problem I could remedy? My shrub is 10 feet tall and standing next to another that is not drooping.
A: The good news is that the drooping of the leaves on your rhododendron is most likely not caused by an insect or disease. This time of year, when the temperatures drop and it is dry and windy, the leaves will begin to curl and start to droop. It is the plant’s way of reducing water loss from the leaves. Some rhododendrons seem to be more prone to this problem than others, but once the weather warms in the spring the rhododendron should recover.
— Jan Gano, OSU Extension master gardener
Tips to ward off spider mites
Q: Our 2-year-old lemon sapling has leaves that are turning white and the tip is dying. It’s currently sitting in an enclosed plastic greenhouse.
A: Your tree most likely has spider mites. Symptoms are white or yellow “polka dots.” The dots are usually the size of a pinhead that can spread to give the leaves a whitish look. Spider mites suck the juices from leaf cells, causing leaves to bleach out.
One treatment is to spray a jet of water on the underside of the leaves. Another is to use insecticidal soap and a third is to use Neem oil. Be sure to use the pesticides exactly according to the label to avoid leaf damage. And be sure the plant is adequately watered before treatment. Spider mites are very common in greenhouse situations and can float on breezes to new plants.
— Pat Patterson, retired OSU Extension horticulturist
Tips to ward off woodpeckers
Q: Woodpeckers are drilling into my house, which overlooks a small park. All my efforts to shoo them away are failing. Any ideas?
A: Total exclusion of the birds from their favorite surface is your best bet, especially over the long-term. But nobody can be outside with an airhorn all day long to shoo them away, and although effective initially, even “scare tape” (mylar strips) can lose its effectiveness once birds realize nothing bad actually happens to them. Instead, tightly-stretched, lightweight, black plastic bird netting — suspending 3 or more inches from the vulnerable part of your building — is the way to go.
— Dana Sanchez, OSU Extension master gardener
Hydrogen peroxide for seed germination
Q: Does a 3% hydrogen peroxide seed soak for 20 minutes enhance seed germination?
A: In general, hydrogen peroxide will boost seed germination, sometimes with only a 1% solution for 30 minutes. Effectiveness will depend on the crop, age of the seed (it works best on older seeds), concentration and time of exposure. Hydrogen peroxide may reduce germination at high concentrations or with long exposure; 3% concentrations are adequate in most cases.
— Ed Peachey, OSU Extension weed specialist
Invasive English ivy
Q: I have the beginnings of an invasion of English ivy in my front yard. Can I get rid of it?
A: English ivy is an invasive plant that produces long vines that root at the nodes. The mature branches produce berries that some birds eat — thereby distributing the seeds. The plants are highly shade tolerant and will root in most soils. The vines will climb trees and, if left unchecked, will kill the tree, thus harming our forests.
To remove the ivy growing in your yard, handpull the vines and then cover the area with several inches of mulch to inhibit new growth. Do not leave what you have removed on the ground as it can reroot. Place it on a tarp to wither, then dispose of it as yard debris. If, after a few months, you see new sprouts, pull them.
Mature leaves are leathery and have a waxy coating, so treating the vines with chemicals is not highly effective. You may have some success treating new leaves when they first appear. Whenever using chemicals, be sure to read the label thoroughly and follow the directions for use and disposal.
— Linda Holmes, OSU Extension master gardener
Invasive toxic arum
Q: We have a pretty significant population of Italian arum in our backyard landscape. We did not plant it and are not sure exactly how it arrived. We first noticed it last summer. Once we identified what it was and how invasive it is, we tried digging it up. It is now starting to come back with a vengeance. Are there any tried and true methods to effectively control and eventually get rid of it?
A: While very attractive, this invasive weed is also toxic. It is difficult to eradicate because it spreads by both rhizomatous roots and seed dispersal.
Arum tends to be resistant to chemical control and, as you have found, it is difficult to dig out. Depending on the size of the area affected, you may find it easier to tarp the entire area and cover it with at least 6-8 inches of arborist woodchips. This smothering may take a year to be effective, but you can place potted plants over the top to create a sort of landscape.
Alternatively, you can excavate the soil down 18 inches, but that is laborious and expensive. .
A licensed pesticide applicator may have access to chemical controls that are more effective than glyphosate (Roundup), but chemical controls have not generally been found to be more effective than the non-chemical methods.
— Lynn Marie Sullivan, OSU Extension master gardener
Ask an Expert is an online question-and-answer tool from Oregon State University’s Extension Service. To ask a question, visit extension.oregonstate.edu/ask-expert.