Sandy's Garden

Why Lilacs Won't Bloom

There are several reasons Lilacs don't bloom

By: Sandy Feather 2007
Penn State Extension

Q. I planted a small lilac two years ago. It is now about five feet tall, but has never bloomed. I have had this experience with lilacs not blooming before. Do you have any suggestions?
  A. Lilacs (Syringa spp.) fail to bloom for a number of reasons. Your plant may not be mature enough to bloom. Many woody plants have to grow for several years before they are mature enough to produce flower buds. You may just have to be patient for another year or so if none of the following factors apply to your lilac.
Late spring frosts can kill flower buds. The plant will be fine, but few, if any, flowers will bloom normally.

Sometimes only part of the bud is killed, and you still have a few flowers able to open on each bud cluster. Warm weather in March had spring moving along a little earlier than normal, until unusually cold weather in April damaged all that tender growth. We have seen a lot of spring freeze injury this year, and lilacs have been among the samples that have crossed my desk.

Planting Location

The location where your lilacs are planted could be another factor. Lilacs need full sun to bloom well. Too much shade can reduce flowering significantly, and deep shade will keep lilacs from blooming at all.  Eight to ten hours of sun daily is ideal; six hours would suffice.

Good Drainage

Well-drained soil is another consideration. Lilacs struggle along in poorly drained soil, or die outright. They expend so much energy trying to stay alive that they might not bloom in that situation.

When to Prune

Maintenance practices can also cause lilacs not to bloom well. They "bloom on old wood," which means they form their flower buds during the previous growing season. If you prune lilacs in the fall, you remove those flower buds, and will not get to enjoy any of that heavenly fragrance. Many spring-blooming plants bloom on old wood, and all should be pruned when they finish blooming in the spring, before they start to form next year's flower buds. You lilac is young enough that you should not have to prune it much yet.

Lilac flower

How you prune will also impact how well they bloom. Lilacs should be pruned by removing the oldest, thickest stems at ground level. The plant will respond by sending up a flush of growth from the roots. Remove weak spindly shoots, and keep well-spaced, robust shoots. This keeps the interior of the shrub open to better air circulation and sun penetration, which makes for a healthier lilac. Lilacs do not lend themselves to shearing the way you would maintain hedges of privet or yew. Shearing removes the tip of each stem, which is where you usually find flowers on lilacs. If your lilac was sheared at the wrong time of year, you would have removed all of the flowers. If you prune as described above, the flower buds on the remaining stems would have been left to bloom for you.

Fertility Level

Lilacs do not require a high fertility level. If you fertilize a young plant heavily to encourage it to grow faster, it may put on leafy vegetative growth at the expense of flowers. Ornamental shrubs and trees generally do not require the level of fertility that lawns, annuals, and some vegetable and fruit crops require. You should always base their fertilization on soil test results.

Soil test kits are available from your local Penn State Extension office for a nominal fee. In Allegheny County, consumer soil test kits cost $12 each, and come with detailed instructions for taking a good soil sample and information to help you understand your soil test results.

Customers ordering multiple kits at one time pay $9 each for any additional kits. (You should take separate tests for lawns, vegetable gardens, shrub borders, etc. since recommendations are based on the crop you tell the soil lab you will be growing on the site). Send a check made payable to Penn State Extension to Penn State Extension, 400 North Lexington Street, Pittsburgh, PA 15208. Write Attn. Soil Test Kit in the lower left corner of the envelope.


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