10 Perennials To Cut Back In August For A Late Flush Of Flowers

August is an abundant time in the garden, but it is also when many plants begin to look tired, straggly, or overgrown. Overzealous perennials may be flopping over the sides of borders and smothering grass or other plants while looking past their peak.

There are numerous flowers to plant in August to make a start on next year’s garden but with a bit of effort, you can extend this year’s growing season.

Even the longest flowering perennials need some attention if they are to keep on blooming, If you have been deadheading earlier in the summer then you should have already been treated to a season of vibrant flowers. However, taking a more aggressive approach to cutting back perennials in August can reinvigorate plants and force them to flower again.

In just a few short weeks, plants will have put on verdant new growth and formed new flower buds. Many plants are quick to recover and reward you with a fresh flush of flowers that will last until the frost.

Bear in mind not all plants can be cut back at this time – plants you should not prune in summer include those that are setting their buds for next year and those that will not rebloom this year and so will be weakened through pruning.

Here we reveal some key plants that you can trim or cut back in August to encourage new blooms, but other perennials can be cut back at this time too. Consider cutting back perennials that have flowered by mid-summer and have begun to look tired, where you can see fresh growth emerging at the base of the plant under the old leaves.

Some of these perennials can be cut hard back to the ground, whereas others will benefit from a light haircut, so check before you make the cut.

1. Cranesbill Geranium

Cranesbill geranium – also known as hardy geranium – is a different species from tender pelargonium geraniums. Come August, plants can look tired and overgrown, but with hard pruning, they will quickly spring back to life and bloom into fall.

Cut the foliage right down to the ground – you should be able to see new shoots already coming through the soil. Within two weeks cranesbill geraniums will already have regreened with fresh new foliage.

2. Lupines

You may have already deadheaded your lupines but if they have finished flowering and begun to fade, then cutting the spent stalks down to near ground level may encourage another flush of flowers.

The flower stem should have died back by two-thirds when deadheading lupines. The flowers die from the bottom up, so only the top third of the blooms should be vibrant.

Where there are new buds growing on the stem, only cut back as far as the new shoot. However, where there are no new buds forming, cut the stem back close to the ground, ideally to a branching point.

3. Delphiniums

Towering delphiniums are a delight to behold in the summer garden but look very untidy when past their prime.

Much like lupines, delphiniums respond well to deadheading spent flower stems. Either trim back to a new flower shoot, or cut the stem right back to the ground – around 2 inches above the surface – ideally terminating at a branching point. This should encourage a flush of late-summer blooms.

Once they have finished, in the fall, cut the plants right back before winter, to prevent the formation of fungal diseases.

4. Lady’s Mantle

Lady’s mantle – or alchemilla – is a popular edging plant and looks gorgeous in cottage gardens.

It’s a prolific self-seeder, so you should deadhead flowers throughout the growing season to prevent it from going to seed. However, come August, if plants have finished flowering and the foliage looks past its best, then cut them right down to the base and water thoroughly.

This hard chop will encourage fresh new leaves to grow, and hopefully, more flowers to form this summer.

5. Coreopsis

With their sunny flowers, coeropses – or tickseed – are such a cheery sight in the garden, so it pays to encourage the longest possible flowering season.

Where the plants are faded, in mid to late summer, cut back coreopsis between a third to a half to encourage vigorous new growth and fresh flowers.

As long as it’s not too late in the season, you can cut them quite far back, though you need to leave some fresh growth near the base. Plants should spring back to life in a few short weeks. This is a particularly useful tactic if the plant has become leggy or weak.

6. Alstroemerias

Alstroemerias – or Peruvian lilies – have an exceptionally long flowering season, blooming from spring right through fall if properly deadheaded. They can slow down for a few weeks in midsummer, so take this time to go over the plants and remove any spent stems.

Rather than cutting them back, the best way to deadhead alstroemerias is to pull the stems out from the root. Simply grasp each stem near the base and give it a good tug.

Within two weeks, the plant should have started producing new stems.

7. Salvia

Salvia is another perennial that offers a long blooming season, often into the fall. However, by mid to late summer, plants are often leggy and overgrown.

For salvias that produce long spires, you should deadhead spent flowers by cutting back to a growth point that has a new side bud forming. This will encourage a new flush of flowers.

Shrubby salvia varieties, such as 'Hot Lips', will benefit from a haircut. Using hedge shears, trim the plant back by around a third. Aim for a nice rounded shape.

8. Garden Phlox

Garden phlox (Phlox paniculata) is a generously flowering shrub beloved by butterflies. Its tall flower clusters – or panicles – can be cut back in August to promote a fresh flush of flowers.

Take individual spent flower stems and cut them right back to a budding point. If you want to propagate more phlox, then take your cut stems and insert them into a moist potting mix. You’ll need to remove the lower leaves first. Garden phlox is an easy plant to grow from cuttings.

9. Yarrow

An attractive, drought-tolerant perennial, yarrow is also an aggressive self-seeder. So, deadheading spent blooms is an important part of plant maintenance.

Cutting the plants back in August will also encourage a late show of flowers and help with drooping stems.

To trim yarrow, take stems right back to near the ground where no new buds are forming, or cut back to a fresh lateral bud. Aim to cut the stem back by at least half to avoid weak, leggy growth.

10. Campanula

Campanula – or bellflower – often yields several blooms over the summer, but for a late-season flush, you can cut back the plant in August.

Aim to cut it back by about a half. Simply grab handfuls of the plant and trim with hand pruners, creating a nicely mounded shape as you go.