Perennials to Cut Back Now for a Second Flush of Blooms – and The Ones to Leave Alone

You may think it's an inevitable slow decline after summer-flowering perennials peak in midsummer. However, when you cut back perennials in August, you can encourage a vibrant second flush of blooms to enjoy.

The likes of hardy geraniums, nepeta, delphinium, astrantia, and lady’s mantle can be coaxed into bonus fall flowers by being cut back in late summer. At a time when even the best perennials can start to look faded and tired, don’t just deadhead but cut the plants back hard. It may feel counterintuitive, but the rewards are worth it.

You need to know when to cut back perennials in August, and also appreciate that not all plants will respond well to this treatment. Here, we look at perennials to cut back in August that you can encourage into a second flush, and those to keep the pruning tools (like these Felco F2 pruners on Amazon that are great for trimming perennials) away from this month.

Hardy Geraniums

Anyone who grows hardy geraniums knows how rampant they are. They are fast-growing and long-flowering perennials that develop into large clumps during the summer.

By August, they can start looking tired and collapse. But you can keep geraniums blooming for longer by cutting the clump back this month. Use a pair of sharp pruning shears or hedge shears and cut the plant right back to a few inches above the soil.

Remove the debris, give the plant a deep watering, and feed it with a balanced liquid fertilizer, such as this 10-10-10 plant food from Walmart, which contains seaweed extract.

It might look harsh, but new growth will quickly appear, and you’ll get a new flush of blooms in little time.

Nepeta

Nepeta, or catmint, is a low-maintenance perennial that fills any space with aromatic grey-green leaves and trumpet-shaped lilac blooms. It works perfectly near the front of flower beds, and you can also grow catmint in pots if you garden in smaller spaces.

When you grow catmint, it can provide long interest. With the right care, this can stretch right into fall. Come August, it can appear a bit straggly, and the stems often start flopping over other plants, lawns, or garden paths.

If you prune catmint back this month to above the first leaf, it will tidy it up, encourage new growth, and provide another round of flowers for you to enjoy.

This trim may take five minutes with a pair of pruning shears or hedge shears. These Fiskars 23-inch hedge shears at The Home Depot are ideal. They have sharp steel blades and soft grip handles for comfortable trimming.

Astrantia

Astrantia produces delicate, papery, pincushion flowers that sit atop slender stems. They come in shades of pink, red, or green-white and have a long flowering season. Indeed, with the right trimming, astrantias are one of an elite group of plants that flower twice or more.

Gardeners growing these cottage garden plants can extend the blooms with regular deadheading. If the plant starts to look tatty in late summer, you can cut it back by half to get a new flush of growth and pollinator-friendly flowers that extend the show well into fall.

Astrantias prefer soil types that keep their roots moist. They can be grown in drier sites, but will need regular watering in hot weather to prevent the leaves from going crispy and ruining any chances of a new set of flowers.

A soil moisture meter, available at Amazon, can be an essential gardening tool to help you judge when to water your garden plants.

Lady’s Mantle

Lady’s mantle, or Alchemilla mollis, is grown predominantly for its soft, scalloped, mid-green foliage. However, don’t overlook its blooms. It produces frothy star-shaped lime-green flowers starting in late spring or early summer.

However, come August, the plant can look untidy with flopping stems and browning foliage. It is a perennial to cut back for fall flowers and this month is an ideal time to do it. When you grow lady’s mantle, you can cut the plant back to the base, and it will sprout new growth within weeks.

Another reason to cut back lady’s mantle after the first flush of flowers goes over is that it stops the plant from self-seeding. If you don’t want lots of new plants sprouting, head out this month and give it a trim.

Delphinium

Delphiniums put on a towering display of blooms in summer. In reality, the season is short and come August, the spikes can look worse for wear. The best solution for this is to cut back delphiniums, pruning the flower spikes right back to the ground.

Cutting off the blooms after delphiniums finish flowering stops the plants looking tatty in borders. It might seem cruel, but the plants can respond with a second flush of flowers.

No matter what types of delphiniums you are growing, trim them, fertilize them, and water them this month. You should hopefully get another round of, albeit slightly smaller, flowers to enjoy in early fall.

If you care for delphiniums, use sharp pruning tools and cut the stalks to two inches from the ground.

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Perennials to Not Cut Back in August

• Peonies - When you grow peonies, they should be cut back in the fall. It is not recommended to cut back peonies in August, as the plants are still photosynthesizing and storing energy for next year’s growth. If you prune peonies now, next year’s display can be badly damaged.
• Asters - Asters are a great plant for fall color. They come to the fore in August and can bloom right through until October. It comes as no surprise to anyone growing asters that the advice is not to cut them back. You can cut back asters in the fall or spring, depending on whether you want to leave the plant for wildlife over winter. As many aster varieties can be a reliable food source for birds over the colder months, and the old growth insulates the plant against winter freezes, I would always opt to cut them back in spring.
• Japanese Anemones - Japanese anemones are another perennial routinely grown for showy performance from August to October. They offer color and interest at a time when summer perennials die down and are adored by many pollinators. However, they do spread through underground rhizomes, so many gardeners use a root barrier (such as this root barrier at Amazon) to keep them contained and prevent them from becoming invasive perennials. If you do grow Japanese anemone, cut them back in late fall or early spring. Don’t do it now or you’ll miss out on their blooms.
• Sedum - When you grow sedum, or Hylotelephium as they are now known, their flowers are rich in late summer and early fall. They are great plants for pollinators in fall, and will attract bees and butterflies in droves. If you cut them back this month, you are missing out on flowers and unnecessarily removing a vital seasonal food source for pollinators in your garden. Your best bet is to cut back sedum in spring, as the dried stems and heads can provide glorious interest and structure in a winter garden.
• Joe Pye Weed - Joe Pye weed, or eupatorium, is covered in nectar-rich flowers in late summer. These tall blooms provide food to several types of bees and many different butterflies. As late summer can be a period when nectar is scarce – it is a gap where summer flowerers have ended and the fall ones are just beginning – you want to retain Joe Pye weed blooms for pollinators. Other perennials that offer pollinators lots of nectar during this gap include black-eyed Susan and coneflowers. These are also plants you don’t want to cut back. You can deadhead them, but don’t prune them completely.

August is a busy month in the garden. While there are lots of maintenance tasks to do – watering, feeding, cutting back – there are also opportunities to take plant cuttings and get new plants for your garden for free. Our guide to plants to propagate in August reveals seven shrubs you can take semi-ripe cuttings from this month.

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