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Great Gardening: You can prune. Just learn how and when.

As I patrolled my yard this week I had the feeling that St. Patrick’s Day must be soon. Yet the calendar says it’s April. In the yard and garden everything is happening slowly, which is not a bad thing.

Gradual seasonal changes are ideal. We often lose some flowers and fruit when a premature warm spell causes plants to plump up their buds and then ‑ boom ‑ a deep freeze hits. This can still happen, but most buds made it through treacherous March.

Freezing nights will keep many plants dormant, so we still have time for pruning. Late winter is the right time to prune most trees, shrubs, grapes and berries. And pruning is something that a gardener or homeowner should learn and feel confident about doing. However…

If I had to name one gardening task that is done worst by the largest percentage of people, I declare it is pruning landscape shrubs and small trees. Yet it is not brain surgery; everyone can learn pruning basics.

Why we prune

Pruning is not “trimming the bushes” or (I shudder) “whacking back the brush.” The primary purpose for this activity is not to shorten all the shrubs, to “get them under control,” or to shape them into geometric forms. (Making topiaries is a whole other art form.) Many plants never need pruning except for rare corrective measures.

Pruning is:

1. Removing branches that are threatening people, property, or the plant’s health or attractiveness. (Always remove broken, diseased, cracked, dangling branches.)

2. Directing future growth, shape, and productivity of the plant. (Remove or shorten branches that obstruct paths, are crowded, look wrong, point inward or rub against another branch.)

Pruning stimulates a response

When you cut or remove a twig or limb, it tells the plant to compensate by putting out new growth somewhere else. If you cut just above a small branch that is growing in a desirable direction, that is good ‑ the plant’s energy will go in that direction. A new branch will be there next year.

This cut-and-response mechanism will help you understand the timing guidelines.

Deciduous shrubs or trees

• Not in early fall: So many people prune in early fall ‑ but it’s basically wrong. Those cuts stimulate new growth exactly at the wrong time of year. The growth will be weak and easily killed off in winter. The plant’s energy was wasted, when the nutrients and hormones should have been used to grow new roots.

Also, cutting before winter opens up wounds that will stay open, letting fungi or insects in when the plant has no protection. After winter the insects or diseases are dead or dormant, and the plant has protective hormones that compartmentalize (form a wall between the wound and the inner plant).

• Late winter: When you make cuts in late winter or early spring the new shoots pop out just when the plant will be receiving rain and sunshine, with a whole growing season ahead. It’s the right time for most plants if they call for pruning.

• Late spring/early summer: If plants flower in spring, such as lilacs, forsythias, or large-leaf hydrangeas (macrophylla), early spring pruning cuts off many of the flowers. It won’t hurt the plants, but the main reason for lilacs is lilac flowers! Prune them after they flower. (In the hydrangea case, cutting those dead-looking stems will destroy the June blooms, but a later flush will emerge from the crown.)

• Early summer - maybe: Some plants produce large amounts of sap and are called “bleeders.” It’s not dangerous or fatal as animal bleeding can be, and we’re happy to use the sap for maple syrup. Trees with active sap production include birch, elm, honey locust, magnolia, poplar, dogwood, walnut and maple.

Do make any important, corrective cuts - bleeding is not harmful to the plant. It can be messy when the trees drip on cars, make your deck sticky, or when insects congregate to suck the sweet liquids. For less bleeding, prune in June.

Evergreen shrubs or trees

• Plants with needles: Spring or early summer is the time, if you wish to shape or thicken the growth of pines, spruces, firs or Chamaecyparis (false cypress) ‑ most of our needled landscape shrubs. When new growth appears (mostly in early summer), clip off one-third of the little shoots to stimulate multiple new shoots. That’s how growers make your Christmas tree thick and fat. It’s best to put landscape shrubs where their mature size will be appropriate and you’ll never have to prune them.

• Broad-leaved evergreens: Hollies, rhododendrons, Pieris can be pruned in late winter, or wait until late spring after their flowers or new growth.

• Boxwoods, yews: Often used as hedges, topiaries, or clipped geometric shapes, these are the forgiving plants, tolerant of clipping and able to grow back throughout the growing season.

How (and how not) to prune

I encourage readers to find a pruning book or online guidelines from universities or arborist organizations, since you need illustrations. These tips just suggest basic concepts:

• Say NO to flat-topping: It’s common but disturbing to see shrubs such as forsythias or viburnums buzz-cut across all the branches to form a wedge shape, perhaps to make them shorter. (Move it, if it’s blocking a window.) The practice is unattractive and wrecks plants’ graceful shapes and flowers.

New growth emerges thickly only at the top of the plant, and soon only bare sticks will show beneath the canopy. Instead, to prune a crowded shrub, reach down inside the plant and remove inner branches, letting wedges of light enter.

• Make pruning cuts ¼-inch above a outward-facing bud or node (where branches grow from,) slanting the cut outward. On larger tree branches, make all cuts just outside the branch collar (the thick area where the branch connects to a trunk or another branch.) Never cut in between nodes or branches, leaving a stub. Die-back and rot will follow.

Good pruning is a science and art. Done right, it makes plants healthier. You can prune, once you know the principles.

*Read Sally Cunningham's story from last week.

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Sally Cunningham is a garden writer, lecturer and consultant.

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