Gardening: Harsh summers may be damaging lilacs

You know we have had some rough weather over the past few years when the nearly indestructible lilac starts having some issues. Several people have contacted the WSU Master Gardener Plant Clinic over the last couple of weeks with questions about pale leaves and tip dieback.

In our climate, lilacs are one of the tougher shrubs in the garden. They are drought tolerant (to a point) and need little fertilizer and little pruning beyond removing old flower heads right after they bloom. In fact, they are so resilient that they still mark the sites of old homesteads all over the region decades after the farmhouses have fallen down.

The most common issue we see here is lilac bacterial blight that results in leaves and shoot tips turning black. Normally this disease pops up in the spring when it’s wet and cool. However, it can also appear when the plants become drought-stressed like they have been for the past two years. We have had a few minor occurrences of it in the Inland Northwest. The common lilac (Syringa vulgaris) and its cultivars that we all have in our gardens can be quite susceptible to it under the right conditions.

Maintaining healthy plants is the best defense against the disease. Improve air circulation around plants by pruning out dense growth. Fertilize the plants lightly in the spring. Too much fertilizer or fertilizer put on too late in the year can over-stimulate the plants. Provide regular water in the driest months to reduce drought stress. Clean up old leaves in the fall to remove disease reservoirs hiding out in the leaves. Lastly, if you have had the disease in the past, the plants can be sprayed in the early spring with a copper sulfate solution to reduce the potential of the disease.

However, what the Master Gardeners have been seeing in the clinic doesn’t quite match the description of lilac bacterial blight. Instead, the leaves appear to be chlorotic. The leaf veins are still green but the leaf is a pale yellow-green with a few brown patches. This doesn’t match any other disease profile and is likely caused by cultural problems around watering, fertilizing and drought stress. Two years of drought probably damaged roots, reducing the plant’s ability to properly take up water and fertilizer, especially during the hot weather we had in August. As a result, the plant can’t support its full canopy of leaves and is now beginning to retire the excess. The plants will probably recover but it is going to take a couple of years and a little extra watering.

The best way to do this is to install a simple watering system for your plants separate from the lawn sprinklers. You need to water the plants deeply and periodically during the dry weather in August and September. Simply wind a soaker hose amongst the plants and set it on a timer for three to four hours every two weeks.

Pat Munts is co-author, with Susan Mulvihill, of the “Northwest Gardener’s Handbook.” Munts can be reached at pat@ inlandnwgardening.com.