If you want bigger blossoms on compact, more refined trees and shrubs, treat those plants like perennials.

Bill Cullina, executive director of the Coastal Maine Botanical Gardens, told the Maine Landscape and Nursery Association at its trade show last month how he and his staff get the strongest impact from plants by a judicious use of saws, pruners, trimmers and lawnmowers.

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Bill Cullina, executive director of Coastal Maine Botanical Gardens, cuts back lilac bushes every three to five years. Shutterstock.com

When gardeners talk about perennials, they usually mean herbaceous plants that die back to the ground each fall and sprout up from the roots each spring.

“Woodies (the term garden professionals often use for trees and shrubs) are perennials, too,” Cullina said, “but they have above-ground parts that live from year to year.”

Many plots at the botanical gardens have a mix of woodies and perennials, all planted more closely together than most people do at their homes. The idea is to give the more than 100,000 people who visit each year a floral feast.

But if the shrubs were allowed to grow unpruned, they would hide the perennials that grow along with them.

To avoid that problem, cut back the shrubs.

It works, Cullina said, because most deciduous species – the ones that drop their leaves in the fall – re-sprout from their stumps. So if the plants aren’t going to act like perennials by dying back to the ground in the fall, you do it for them.

Some can be cut yearly, while others should be cut less frequently, up to five or seven years apart, depending on the plant and why you are growing it.

For most plants, he says, wait until they are well established – up to five years in your garden – to start this pruning regimen.

Cullina believes in cutting back hydrangeas hard and often.

Hydrangea arborescens (the most popular cultivars are “Annabelle,” “Bella Anna” and “Incrediball”) should be cut back to the ground every year.

“You get the leaves out” after the cutback, he said, and “go back in the spring and mulch.”

People who enjoy the flower heads for winter interest could do the cutting in March, but cleanup is easier if you do it in the fall.

“If you want the large basketball (sized) flowers, the best way to do it is cutting them back,” he said.

You can also cut back hydrangea paniculata (PeeGee, “Limelight,” “Pinky Winky” and others) every year, but not quite to the ground, leaving slightly more existing growth each year.

Cullina will sometimes use hedge trimmers for the initial cut, and pruners to thin out deadwood below that cut.

With the “Endless Summer” series, which blooms on both this and last year’s branches, he cuts them to the ground about every third year, sacrificing the early blooms that year to keep the plants more compact and make the blossoms larger.

As companion plants for hydrangeas, Cullina uses globe alliums (tall ornamental onion plants that are have striking softball-size purple blooms) all through the bed for spring interest. When they go by, the hydrangeas will be in bloom, and in some beds, baptisia blossoms after that.

Spirea blooms on the current year’s growth, so it is almost an herbaceous perennial that you can cut back to the ground every year – and it blooms better if you do.

Buddleia, or butterfly bush, is marginally hardy as a shrub in Maine, but the roots stay alive and will re-sprout. Cullina cuts them back anyway each year.

Physocarpus, or ninebark, is a native shrub that blooms on last year’s branches – but many of the newer varieties are grown more for their colored foliage, which ranges from yellow to dark red. If you don’t care about the blossoms, cut the shrub back to the ground every year, and it will shoot up lushly each spring. If you want the blossoms, cut a third of the stems down to the ground every year.

Diervilla lonicera, or false honeysuckle, is the type of shrub that you put in a mass next to your driveway where the plow will dump all the snow. It has colorful foliage and yellow blossoms and grows 3 to 5 feet high, depending on the type. If the plow wrecks the plants, no problem – Cullina runs a lawnmower over them every spring. He treats ceanothus – a plant I had never even heard about – and comptonia (or sweetfern) the same way.

Plants he cuts back every three to five years just to rejuvenate them include lilacs, large-leafed magnolia, winterberry holly, bottlebrush buckeye, chionanthus (or fringe tree) and witch hazel.

Now, most homeowners won’t want to follow the botanical gardener’s techniques. Many love the look of full-grown shrubs and don’t worry about hiding perennials intermixed with the shrubs. And all that pruning is a lot of work.

That said, the effort could result in more blossoms that are bigger and better – so it’s something to think about.

AND THE ENVELOPE PLEASE

Also at the Maine Landscape and Nursery Association trade show, Emily Buck of Plants Unlimited in Rockport, founded by her father, Hammond Buck, was honored as young nursery professional of the year.

And Jeff Marstaller, co-owner of Cozy Acres Nursery in Yarmouth, which added a greenhouse that requires no external energy to run, was named winner of the Al Black Award as the outstanding Maine nurseryman for 2015.

Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer gardening in Cape Elizabeth and can be contacted at 767-2297 or at tomatwell@me.com.