Garden Q&A

Question: What causes trees and perennials not to bloom? We have a seven-year-old Styrax japonicus (Japanese snowbell tree) in a relatively small West Philadelphia backyard. It is now 20 feet tall and wide, appears to be quite healthy, and has bloomed for three or four years. This year, however, in May it bloomed only a very little on a couple of branches, and then stopped. Our phlox (tall, white) also hasn't bloomed yet; it's usually a June bloomer, but I'm assuming that everything is late this year because of the weather. Nothing has changed, except that the yard has become shadier.

- Carole A. Parker
Answer: All sorts of things affect blooming, some of which may have taken place months ago, some more recently. Some you can control and some are just the ways things are.

Styrax forms buds the previous year. If it didn't form a full complement of buds last summer/fall, nothing will change this year's performance. The limited May flowering may have been a case of some precociousness on those branches, with the balance waiting to open in more timely fashion (snowbells are late spring bloomers). But if those few May blossoms are all there are for this year, the "problem" goes back to bud formation last season, which may have been a function of how much rain there was in the necessary time frame. (A current-year circumstance that is illustrative of this phenomenon is that many rhododendrons are already setting buds for next May's flowers - which I surmise is a function of the substantial rains of June. Parched years can have the opposite effect.) Conceivably some of the styrax buds were frozen over the winter. It is a reliably hardy tree, but last winter did have a deleterious pattern of three significant cold snaps with milder weather between, though I doubt that's the cause. There's also the possibility of excess nitrogen (from fertilizer, including lawn applications) at the wrong time; nitrogen promotes foliar growth at the expense of flowering.

As for phlox, you're both getting ahead of things and correct in surmising that some perennials are a little behind schedule due to the gloomy late spring weather. My Phlox paniculata 'David' (probably the best tall, white strain), which gets a bit more shade than is optimal, has not yet bloomed.

And there's that dreaded word again: shade.

Most gardeners underestimate the effects of increasing shade over the years. It's like watching children grow; a seldom-visited relative always points out, "You were so small the last time I saw you," while both the parent and child are basically attuned to recent incremental change. A photograph of your garden from a decade ago might really surprise you.

A number of years ago I planted three peonies in a sunny spot. But I had also planted an 8-inch Magnolia denudata not far away, just south of the peonies. The magnolia is now 20 feet tall and the peonies are in shade - and blooming feebly each year. I have determined a new, full-sun site for them and will move them in September. You may need to do likewise with your phlox, which wants a very sunny site to perform best. If you don't have that option and the current site really is too shady, you may need to grow a more shade-tolerant plant instead - or cut down the shade trees (just joking).

And like children, plants do not perform identically every year, no matter what the shade situation. That's nature for you.

- Michael Martin Mills