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Learn the '3-year rotational' and prune lilacs & other flowering shrubs like a pro

A gloved hand holds pruning shears up to a branch with white flowering blossoms.
If you prune too early, too vigorously or too late, you might be lessening your chances of seeing blooms next spring.

We've enjoyed some beautiful spring flowering shrubs this season, beginning with forsythias and lilacs and now nine-barks and weigelas.

In order to ensure more beautiful buds next year, now is the time to think about pruning them.

Right after they're done flowering, you've got about a six-week window of pruning time, up until about mid-July.

And after that window closes, those flowering shrubs get busy setting their flower buds for next year.

If you try pruning in later July or into August and September, you're essentially pruning off any flowers that might have blossomed next spring.

Rhododendrons, ninebarks, lilacs: Depending on the size of these flowering bushes, pruning can take on a couple of different methods. Some may seem extreme but they will keep the size in check and keep it flowering from year to year.

If you planted a flowering bush years ago and have not been pruning it season after season, it may have grown to enormous proportions.

For rhododendrons and ninebarks gain some control and space back by cutting them all the way back.

They will sprout out from the old growth and from the root growth. It will take a couple years to get to a nice shrub size again but it will be a manageable and will flower once again.

Pruning overgrown lilacs is a bit different. Instead of just removing a ton of branches and stems - which may result in no blooms for a long, long time - try the "three-year rotational method."

Each year, remove a third of the lilac branches and stems then leave the rest there to flower.

Watch Charlie Nardozzi prune a giant lilac tree on Ben & Marian's proper in Berlin! Scroll to the 20-minute mark to see all the lilac-pruning action on All Things Gardening LIVE here.

By the end of the third year, you'll have reduced the overall size of the shrub to a manageable one. And all the suckers and new growth from the first year will now be old enough in that third year to flower.

If your lilac bush is already a manageable size and you're just trying to prune it back a bit, cut back to a side branch or two buds with leaves on them. You can cut it back six inches to a foot.

Weigela: These cascading flowering shrubs should be pruned now, too. To do so, take individual stems all the way back to the main part, or the crown of the plant. Doing so will help the weigela keep its nice weeping shape.

Q: A friend just moved to Vermont from Pennsylvania and there, Bishop's weed is not a problem. But here it has invaded all her gardens. Why would it thrive here and not in Pennsylvania? Is it a difference in the soil, the weather, the snow cover? Any ideas for how to control it? - Lorraine, via email

Gout weed, Bishop's weed, snow-on-the-mountain or aegopodium is a very invasive weed. Native to Northern Europe and Asia, it thrives in cooler climates.

That might be one reason why it didn't really thrive in Pennsylvania but really takes off in Vermont's cooler temperatures. It also loves more moisture and shade, of which Vermont has plenty.

It is very fast-growing, much to the detriment of other native species around it.

Got knotweed? More from All Things Gardening: With roots that can run 10 feet deep and a zombie-like afterlife, knotweed is a difficult invasive to control.

And this weed has an extensive root system, so if you're trying to control it, you'll have to be just as aggressive!

Keep mowing it down, over and over (and over!) through the summer. You'll eventually exhaust the roots.

Then throw a black plastic tarp over the area and try to smother it. Come next spring when you pull the tarp off, you'll see a few of the weeds popping up but you will have probably eradicated quite a bit.

Q: I found patches of stilt grass seedlings in my garden and I'm wondering, what is the best way to get rid of it? I've read it's very invasive and hard to get rid of. - Claire, via email

A: Yes, stilt grass is very tough to get rid of. And it's a relatively new weed to Vermont.

Stilt grass is really prevalent down in the mid-Atlantic states and in southern New England where it's warmer.

This grass doesn't like cold weather but with climate change, data shows that a couple of years ago, stilt grass is now popping upin southern Vermont counties like Windham and Bennington, and even in Rutland. It may even be growing in the Champlain Valley by now.

Stilt grass grows kind of like a Japanese knotweed, with bamboo-like shoots that come up one to three feet tall.

More from Vermont Public on another way to think about an invasive plant. Using what's available: Vermont herbalists turn an invasive plant into medicine

It spreads by underground roots, but it's an annual, so it seeds and that's the key to controlling it. Cut it down and keep cutting it down so it doesn't go to seed.

Stilt grass also roots from its stems. It will grow and spread really quickly and crowd out native plant species.

It's not an easy one to control but you can always try!

All Things Gardening is powered by you, our audience! Send us your toughest conundrums and join the fun. Submit your written question via email, or better yet, leave a voicemail with your gardening question so we can use your voice on the air! Call Vermont Public at 1-800-639-2192.

Listen to All Things Gardening Sunday mornings at 9:35 a.m., and subscribe to the podcast to listen any time.

Mary Williams Engisch is a local host on All Things Considered.
Charlie Nardozzi is a nationally recognized garden writer, radio and TV show host, consultant, and speaker. Charlie is the host of All Things Gardening on Sunday mornings at 9:35 during Weekend Edition on Vermont Public. Charlie is a guest on Vermont Public's Vermont Edition during the growing season. He also offers garden tips on local television and is a frequent guest on national programs.