Boosting lilac bloom

K.L./Flickr

K.L./Flickr

Many of your lilacs, I’m hearing, have been disappointingly shy of bloom. The tales are sad and the gardeners despondent about the lilacs that bloomed once — three years ago — and not again. Without their Rubenesque trusses of fragrant flowers, lilacs are just another big, green bush, no? Well, I’m here to help.

Let’s think first of the spot you’ve chosen. Lilacs need lots of sun to produce flowers, at least six hours of direct light daily. Perhaps, in your leafy suburban neighborhood, the trees have grown up around your lilac since it was planted, depriving it of sun. There’s precious little you can do, because a lilac wants what a lilac wants. You can: a) move the lilac; b) trim the trees; or c) plant another lilac in a more open location.

Maybe, if you’ve planted it recently, your lilac is just immature. Young plants need two or three seasons to establish a decent root system before they hit their flowering peak. Meanwhile, you can help it grow big and strong by keeping it watered, since these plants suffer in a drought. And that’s not a sprinkle, but a good, deep soak every week or so during the first season if rain doesn’t fall.

As with all shrubs, and especially newly planted ones, you should keep groundcovers, grass and weeds away from the base of the plant, since they rob it of water and nutrients. A nice blanket of mulch helps to retain moisture, but don’t pile it up in a volcano-like mound around the base, or you’ll be inviting mice to nibble the bark away on a cold, winter’s day when the mousy larder is bare.

Although lilacs are easy to grow and hard to kill, they wouldn’t mind a little fertilizer, especially in the early years. But don’t overdo it, especially with high nitrogen fertilizers, the nitrogen content being indicated by the first of three numbers describing the formula. Too much nitrogen -- too much fertilizer in general -- will get you big, healthy, deep-green leaves but not much flower power.

Nutrients of any kind, already in fertile soil or added by your hand, will go wasting if your soil is too acidic. Lilacs are lime-lovers, preferring soil with a pH reading of 6 to 7.5 on a scale where 7 is neutral (higher numbers are more alkaline, lower ones more acidic). The pH factor is like a gatekeeper, and if it is woefully wrong, it can prevent a plant from making use of soil nutrients, no matter how abundant.

If you live in a pine and oak woods, or on thin, sandy Pine Barrens soils common in the southern half of the state, if azaleas and rhododendrons thrive at your place, chances are your soil is on the acid side. The smartest way to check is to get your soil tested by your county office of the Rutgers Cooperative Extension Service. But here’s a general guide: If you need to lime your lawn, you’ll need to lime your lilacs.

The ideal lilac                                              Brian Pettinger/Flickr

The ideal lilac                                              Brian Pettinger/Flickr

I inherited some truly ancient lilacs, since fallen prey to borers and rot and replaced outright. I did find that a serious pruning and a single dose of superphosophate (available at garden centers) kick-started them into blooming more prolifically. Follow the dosage recommendations on the package and water it in well. Phosphate is the element most associated with the production of fruit and flowers.

Speaking of pruning, prune you must, but do it in a timely fashion. If you have been trimming back your lilacs in fall or winter, there’s a simple reason why you have no flowers. You’ve cut them off, silly. Flower buds are formed in early fall, primarily on the current season’s growth. To stimulate the growth of new wood, which is inclined to flower, prune immediately after the lilacs have bloomed in spring — like now.

Lilacs are suckering bushes that will form an ever-widening thicket of branches, and it can be daunting to approach an overgrown 15-foot specimen armed only with a pair of loppers. Don’t be faint-hearted, since it’s hard to prune these puppies too much.

Preserve a reasonable number of strong, healthy limbs to serve as a framework, and then pare away crossing branches to open the center of the bush to light and air. Head back those tall limbs waving overhead by about a third to put flowers closer to your nose. Once you get an annual regimen going, you’ll want to get the saw and cut to the base one or two of the oldest, thickest branches to allow younger ones to develop.

Lilac suckers can become substantial new shrubs that can lead independent lives elsewhere, if you like. Sever the umbilical root attaching such offshoots to the mother plant, dig the daughter plant up with a substantial root ball and set it in somewhere else. If you don’t need another lilac, you can try to quell sucker production not by pruning them off at the soil line, but by digging down and tearing, rather than cutting, the sucker away.

As for deadheading spent flowers, experts are divided on whether this will boost the blossom quotient next season. This can be quite a chore on a big, sprawling bush; I just cut flowers for the house with abandon and leave it at that. Yearly pruning to promote new, flowering wood is really more important than clipping every faded flower truss away.

If you’ve done everything right and your lilac still won’t bloom, here’s a final trick I learned from Eric Welzel of Fox Hill Nursery in Maine, a specialty lilac grower: Scare it. No, no — not by leaping out from behind the shrubbery and shouting “Boo!” Make a scrape in the bark 2 or 3 inches long near the base of the trunk right after blooming season.

“Nothing fancy,” he says. “Just a boot scrape will do. This “shock method’ almost always works for me.”

The theory is that by stressing the plant, you frighten it into producing seeds so that it — or its progeny — will survive the threat. And what precedes seeds? Flowers, of course. Just don’t overdo it, since girdling the trunk (removing a ring of bark completely around the circumference) will almost certainly kill it.

And then you’ll have no flowers at all.