Judging strictly from the number of customer inquiries nothing, and I mean nothing, is as looked-forward-to in the spring garden as the arrival of lilac flowers. When the highly anticipated floral extravaganza fails to materialize, the customer's crushing disappointment is followed by plaintive questions: "What happened? What did I do wrong? Will it help if I fertilize? Isn't there something I can do?" Gardening self-doubt runs rampant.
Lose the guilt (those of you who are sure they are somehow at fault) and look at possible explanations for lilac flower failure. In my experience this is a much more common problem with what I call the "grandmother lilacs" (Syringa vulgaris - Common lilac) than the dwarf lilacs (S. meyeri and S. patula). That is, unless poorly timed pruning is an issue and then the latter may fail to bloom, too (Reason #9).
- You planted it in the last year or two. It was a small container grown plant. It's growing quite nicely, but there are no flowers. Plant is simply too young to set flower buds.
- You planted a more mature balled & burlapped plant. It's growing quite nicely, but there are no flowers. The plant is directing energy to regenerating roots left behind in the field when it was harvested. The larger the plant, the longer it will probably take to produce flowers.
- The plant is receiving more shade than it should. Lilacs like a minimum of 5 hours (more is better) of direct sun. Direct doesn't mean filtered through trees. If you love lilacs and want to be successful assess the site for sun before you buy. Sun, sun, more sun, please.
- The plant is in a bed near the lawn. The lawn is being fertilized three or more times a year with a high nitrogen fertilizer. The fertilizer is being broadcast and flung into the bed with the lilacs. Lilacs, like many plants, respond to nitrogen by producing lush stem and leaf growth. In that hormonal state lilacs are not in a flower producing mode. Upon questioning I find this happens a lot.
- It was an overgrown plant and someone did a hard rejuvenation pruning, cutting it back quite dramatically. It's growing back with lots of new young green stems. Same as #4. You'll have to wait for it to slow down, switch gears back to a flowering state again.
- The previous summer was really hot and dry. The plant didn't get supplemental water when it was under stress and chose resource conservation (read survival) rather than forming new flower buds.
- You had a lilac that had to be transplanted for some reason. It was a blooming size plant, but the root ball didn't hold together very well. So the shrub was, shall we say, stressed. That was two years ago and it's still not blooming. Same as #2, lilac in recovery mode.
- The plant flowered like a champ last year, you've never seen it so beautiful. Deadheading (removal of spent flower) wasn't done. No flowers appeared the following year. Explanation: The shrub was trying to produce seed from all of those flowers. Therefore, energy was spent in that pursuit, at the expense of this year's flowers. Deadheading might have made the difference between some bloom and none at all!
- In addition to all of these the biggie is pruning too late in the year. Because lilacs are spring flowering, they're blooming from buds formed the summer before. If your ______________________ (multiple choice, pick one: spouse/offspring/landscaper/gardener) pruned in July or later, flower buds that might have been forming were cut off. Bloom potential... lost. This happens a lot. Try to do any pruning or shaping ASAP after flowering is finished.
For at least nine reason lilacs can be like many Chicago sports teams, "Just wait 'til next year."