How (And When) To Cut Back the Tall Late Bloomers

Really the title should simply read WHEN to cut back the too tall late summer bloomers to prevent flopping. Most gardeners already know how to cut stuff, but this is part of our How To series. The rest of these instructional posts can be seen on the side bar, Categories, How To. (Shown above: Vernonia gigantea, backed by Japanese blood grass, September, 2010)

A list was started several years ago of those plants that appreciate having their stalks cut by no more than half around mid-May here to induce branching and to prevent needing to be staked later on. If we wait too long to cut them, that branching results in flowering so late that many are threatened by frost before being allowed to fully open. If the chore is done too soon, the plants will still be too tall come bloom season and will fall over with the weight of the flowers. (Shown above: Sedum ‘Matrona’, September, 2010)

Number one on the list, and if we only get around to cutting one genus, it must be the Vernonia gigantea syn V. altissima that came with the property. As you might guess from the name, this is a reach for the stars plant, growing to twelve feet in height when mature and happy. Even with the cutting it will be well over six feet, but with more branching. There are other, shorter Vernonias, like V. lettermanii ‘Iron Butterfly’ that should need no such treatment. Or at least that is the assumption since this is its first full growing season here. If need be, it will be added to the cut list. (Shown above: Vernonia gigantea in bud, August, 2009)

Next up is the passalong species Phlox paniculata. This hard working late summer into fall bloomer helps give color to the garden and offers nectar for hummingbirds and pollinators at a time when there are few blooms about. Some of these will be allowed to bloom with the daylilies, to help unify the disparate hues of the daylily hill, and will be deadheaded after bloom with hopes of more flowers that can beat the late October frosts. But most will get a whack by half to present color later on. (Shown above: Phlox paniculata in front of Rudbeckia lanciniata, August 2009)

Asters and Goldenrods, the tall ones that came with the property and the New England Asters, shown above, that were ordered as plugs from Seed Savers years ago, will stand up straighter and offer bushier and therefore more floriferous delights for the pollinators in fall if shortened now. The general rule for what to cut is anything getting tall right now that blooms after the fourth of July. Some seedlings, as many of these are, will not need cutting this year. Look for strong, stout stems on eligible cuttees. (Shown above: Aster novae-angliae, June 2010. I do not recognize some of the name changes made by taxonomists who show contempt for the human race with the names they choose to replace perfectly sensible ones.)

Mums are next on the list. We do not cut the Sheffield Pink variety, known here as the Sheffies. They are sprawlers anyway, but the tall button yellow unidentified mum and those growing on the daylily hill will be shorn. Those growing with the daylilies need to be kept shorter anyway to allow for the shorter Hemerocallis plants to be better seen at their peak. White Daisy, Soft Yellow, Ryan’s Pink and Pink Grandchild will get this treatment.

Last year the Japanese Anemones were cut all the way to the ground in a fit of control freakism since they were blocking the pathway. That resulted in no blooms. This year they may not be cut at all. To cut or not to cut is still a work in progress on this one. Perhaps the best compromise is to cut them quite short where they are growing along the path edges and between the step stones.

Other plants that might get a trim are the tall Sedums, Gaura and Verbena bonariensis, shown above. I have not found deadheading to be of much value in prolonging bloom of Echinaceas or Monarda here, so will leave those to form seedheads for the birds and winter interest.

You may have different plants that need a haircut at a different time than those mentioned here. Results will vary according to climate, so you can make adjustments as needed as to the timing. But cutting the talls whenever is right for you will be a garden chore that will have big payoffs in the months to come. Knowing which ones to cut is the key, as is when to cut. Good luck out there! Snip, snip.

For other How To posts written by Fairegarden, look for How To on the sidebar page listing or click here.

Frances