Question
I have 2 lilac trees which are of concern. Before the leaves were thick and it was impossible to see through from one side to the other.  I generally trim off the blooms once they have died.
For the past 2 years, the leaves are getting sparce and it easy to see through the tree branches.  No burned leaves, occasionally curling ones but mostly it is a lack of fullness.

Should i trim the trees way back?  They have great sentimental value to me and I'd like to do whatever I can to save them.

Answer
Jennifer,
I'm assuming that you have the spring flowering lilacs, known to us plant geeks as Syringa vulgaris, yes? I ask because there is another large tree form of lilac that blooms later in the summer (aka Syringa reticulata) so I want to be sure we're on the same page.

If you are talking about the lilacs that have purple, lilac, pink or white flowers in the spring, I'd say the following:  First, you want to be sure that the soil around these plants is alkaline...have a soil test done at your local cooperative extension service. I don't know where you live, so it's hard to advise, but in general people in the NE United States have acid soil and those in the West have alkaline soil...in between, it could be one or the other. Lilacs like a neutral to alkaline soil so a test of the pH will tell you what to do.

Secondly, be sure to keep the soil around these plants alive by an application of organic matter once a year. A bag of composted manure, spread to an inch thick, around each plant every year will work wonders. Here's how I think of it: in nature, there is no "fertilizer fairy" sprinkling nutrients and soil amendments every year. In nature the entire system works by organic matter (leaves, stems, branches etc) falling onto the ground and rotting around the plants.

We gardeners, however, tend to clean up. We take away the falling debris. So it makes sense that we need to add back some composted organic matter to replace what the plant is expecting in natural circumstances. So some composted manure spread over the ground after you apply some organic fertilizer is perfect.

Finally, know that lilacs can be rejuvenated by pruning one or more of the oldest stems down every year - if you have five older trunks or stems, for example, cut one of them down this year to about 18 inches tall. This will produce many new stems around that plant. This can rejuvenate older plants when in combination with the composted manure, organic fertilizer and proper pH of the soil.

Should you have actual summer flowering lilac trees (always white) let me know and we'll begin again.

I hope this helps,
C.L.