Is It Time to Prune Yet?

Daunted by when to prune which flowering shrub — and how? This short refresher course could raise your confidence level.

I know few more masterly pruners than Jeff Jabco, who in June marks 30 years at Scott Arboretum of Swarthmore College in Pennsylvania, where he is the director of grounds and coordinator of horticulture.

Mr. Jabco’s team maintains not just the 300-plus-acre arboretum, with its more than 4,000 kinds of ornamental plants, but the entire Swarthmore campus. I asked his advice on getting the most from lilacs, hydrangea — and yes, even that overgrown forsythia out front. (This interview has been edited and condensed for clarity.)

You want to determine if the plant is flowering on old wood — branches that formed last year, with flower buds formed on them at that time — or on new wood, like Hydrangea paniculata. Typically with spring bloomers, you prune after bloom, unless it’s a fruit tree, in which case you would prune before bloom.

With a fruit tree, you’re pruning so the tree doesn’t have too much fruit, and so fruit that does form has good light and air. Fruit pruning doesn’t follow the rules for ornamentals.

Right. On our Aronia, though I wouldn’t prune them now, I did remove crowded, brown or crossing stems. I do that health pruning early on, even in early bloomers that I am not going to otherwise prune — unless maybe if a branch is sticking out too far, and it needs a little correction.

That’s as opposed to a lilac, where when you prune you’re thinking about blooms for next year, not about letting seed capsules develop. (Nobody wants to eat those.) And when lilac seed capsules hang on they look bad, and the plant wastes energy.

With forsythia, you want it to have that big, arching look. You’re not really supposed to be shearing that in summer. This year, when it has finished flowering, cut some of oldest stems to the ground. The same holds for Physocarpus and Spiraea. For any of these shrubs, if they’re too big, tip them back here and there a little, carefully, but don’t shear them.

Just shearing back ruins the form of what the plant naturally is, like that forsythia or viburnum. If you shear only a few inches off, the underlying wood gets thicker and the shrub gets denser each time, and you eventually cannot keep it in bounds anyway.

It’s always safe to cut back to lateral branches — meaning from the tip of the stem that you want to reduce in length, cut it back to a side branch. This type of pruning can gradually reduce size, but it doesn’t change a plant’s natural growth form. But if you didn’t gradually trim a bit each year before it got too big, the shrub gets bigger, thicker, denser — and the only option may be to rejuvenate the whole plant, to cut all the stems down.

To completely rejuvenate a shrub (and not all shrubs can withstand it), the stems are cut back completely, to typically less than one foot. This removes all of the foliage (if it is an evergreen like a holly, boxwood or yew). This is done in very early spring, after threats of severe freezing are past, but before new growth starts, so the plant’s energy goes into development of shoots growing out of the remaining bare stems.

There are some plants you always want to maintain as young stems because of the look. For instance, the twig dogwoods, like Cornus sericea, and twig willows. Otherwise, they develop their older bark that isn’t red or gold, and that’s another example of when you’re pruning for aesthetics. With the red stems of Cornus sericea, I probably cut 80 percent of the total number of stems to the ground each year.

Whether the smaller ones that flower later or the tall, common lilacs, Syringa vulgaris, it’s still the same idea: You want to deadhead them just after bloom to prevent energy going into those seed capsules. Plus, they still need room for vigorous new stems to develop.

Especially with the old ones, proper pruning helps prevent powdery mildew. If you let everything that comes up from the base develop, no air and light gets down into the plant. Old lilacs will benefit from a bit of low-nitrogen organic fertilizer each year, and in acidic soil, an application of lime or pulverized limestone every three to five years.

If a lilac has been in a while and is vigorous and healthy, I would still never take more than 25 percent of the stems out at the base. If there are a dozen, I might take three. If the plant is weaker, or old, I wouldn’t cut out as many — perhaps one or two.

Some species bloom on old wood, some on new — like summer-into-fall-blooming H. paniculata. All I really do on those is cut off remains of last year’s blossom, back to live buds, but you can prune back farther. However, select a cultivar geared to your space; don’t try to shape a 12-footer like Tardiva into a dwarf like Little Lamb.

Hydrangea arborescens Annabelle blooms on new wood, but I cut it back really hard in early spring, to four to six inches, so its flower-heavy stems don’t flop in the rain later on.

With the often-blue-flowered macrophylla, or mopheads, which bloom on old wood, I cut out up to 15 to 25 percent of stems older than three or four years, plus weak young ones, none of which will develop into good-flowering stems. Then you tip back the remaining ones to a pair of strong buds — we always get some tip dieback that doesn’t harden off in fall, or gets hit with a late freeze.

Even though, technically, they flower on old wood, oakleaf hydrangeas (H. quercifolia) should be pruned like Hydrangea paniculata: That is, they flower in summer, and they generally don’t require too much pruning.

I would still prune like other mopheads.

After things leaf out that you pruned earlier, go back and have another look. There will be a stem that didn’t flush, or a bud that didn’t break after all. This is a good chance to do that detailed, picky pruning.