Growing things: How to get lilacs to bloom

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Q: As a long-time fan of your columns and advice, I took notice of your May 4th article. I planted two dwarf Korean lilac bushes 15 years ago. They flowered nicely. As age took over, I am getting fewer and fewer blooms. Leaves come and the bushes are green but very few blooms. I find the same with a double-flowering plum bush I planted around the same time. Leaves but few blooms.

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Is the age of these bushes a factor? Is there a type of ‘miracle’ product [ie fertilizer] I should add either directly or via water to get them to bloom better?

Appreciate any advice you may have for me.

A: My first question would be the location of these shrubs. Are they close to a lawn and might they be getting exposed to lawn fertilizer. If so the high nitrogen in the lawn fertilizer might be the reason the shrubs are leafing out nicely but not blooming.

If this is the case try to avoid having the lawn fertilizer drifting in the vicinity of the shrubs. The other question would be has the lighting changed for these shrubs? Over the 15 years are they now shaded more than they used to be? If they are not getting at least 6 hours of sun a day this might be the problem. If the location is not the issue, then we need to look at other possibilities.

Are they getting too much water? Lilacs especially will not bloom if they have water-logged roots. They also don’t like it too dry. Are you pruning the plants at the wrong time? Prune immediately after blooming. Don’t wait until the fall or spring. Are you deadheading the spent flowers? This will encourage bloom.

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Your plants are 15 years old and yes, age may be a factor. Lilacs bloom best on younger wood. Blooms can be sparse if your lilac consists primarily of old wood.

You might need to do a rejuvenation pruning. In early spring, remove 1/3 of the stems or branches, cutting them back to 15-30 cm above the ground or main trunk. Cut side branches back to the main stem. In the second year, cut 1/2 of the remaining old wood, and remove all remaining old wood the third year.

Remember that this is a drastic step that will take a few years for the plant to recover.

Unfortunately, there is no ‘miracle’ fertilizer that can turn your shrubs into blooming wonders. 10-10-10 is a standard fertilizer for lilacs and double-flowering plums.

Every week, Growing Things Outdoors runs online at or, if you prefer an epaper format,

Learn more by emailing your questions to [email protected], reading past columns or my book Just Ask Jerry. You can also follow me on Twitter @justaskjerry01.

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