Divide These Perennials in May to Multiply Your Favorite Plants – and Fill Borders for Free
The spring window for dividing plants is closing, so we advise acting fast
Herbaceous perennials fill borders with flowers, form, and textures, and can do so for many years. To keep them performing at their best season after season, you should divide them every few years – and May offers a final chance to divide summer-flowering perennials.
Dividing perennials is a quick, simple task for spring. The conditions are ideal, with the soil warm and moist, and the plants are full of energy at the start of the growing season. Splitting perennials early in May gives them time to establish before the summer temperatures arrive. Dividing plants not only keeps clumps vigorous, but is a sustainable task that gives you new plants for free, rather than spending money on new ones to fill your borders with blooms.
1. Hostas
Hostas are really easy to grow and make stunning ornamental additions to any flower bed, with their dramatic foliage coming in a range of colors and sizes.
As clumps get congested over time, the plant’s performance can suffer. And if you want to grow hostas that are strong enough to hold out against slug attacks, your clumps need to be as healthy and vigorous as possible.
Thankfully, hostas are easy perennials to divide, and they respond well to this operation. Divide hostas every four or five years, either in the spring or early fall – though I do prefer it as a springtime task.
Once they are starting to grow and you can see buds appearing, lift the clump, remove lots of the soil, and divide it with a spade or sharp knife (like this garden knife from Amazon).
Plant the hostas into your borders as quickly as possible, and water them in well.
2. Yarrow
Yarrow (Achillea) is a long-flowering perennial with flat-topped blooms made up of lots of little flower heads. The flowers come in a range colors and are nectar-rich, which makes them great plants for pollinators.
When you grow yarrow in your flower beds, clumps do spread. This can mean they end up with dead centers, get congested, or spread further than you want. The best way to prevent this issue is to divide clumps every 3-5 years.
Do this in spring, just as new growth is appearing. Use a sharp spade to dig around the clump and lift it from the soil. Shake off the excess soil, and cut the root ball into sections. Depending on the size of the clump, you can get up to 4-6 different sections – just ensure that any division has a decent amount of roots and shoots.
Plant the new sections into gaps in your borders, or into containers if they are smaller divisions. Water the new plants deeply to help them establish in their new home.
3. Daylilies
Daylilies (Hemerocallis) are showy perennial plants. Despite their name, which comes from the fact that individual flowers last just a single day, clumps can continually produce lots of vivid blooms for around a month or more.
You can grow daylilies of many colors or patterns, and the plants will form large clumps over time. As the clumps enlarge, it causes congestion, leading to weaker growth and fewer blooms. For healthy clumps and the best flowers, it is advisable to divide daylilies every four or five years.
Springtime is an opportune moment to divide the clumps, and then you can plant daylilies into your borders for future displays. However, it is worth remembering that any new clumps may take one or two years to start blooming.
Daylilies do produce fibrous roots. This means you may need to deal with a mass of these roots if you haven’t divided them for a few years.
A sharp knife will be your best friend here; use it to slice through the clumps. Cut them into sections with established stems and roots, and replant with the crown an inch below the soil surface.
4. Asters
Asters come to the fore later in the season, blooming in late summer and fall. As fall-blooming plants, the ideal window to split these perennials is in spring, when new growth is appearing for the season ahead.
Over time, clumps of asters can get leggy, and they are another perennial that suffers from dieback in the center of the plant as it spreads outwards. As this central portion of the plant is exhausted, the best plan of action is to divide your asters every 3-4 years and replant divisions of the vigorous outer sections of the clump.
To divide any aster varieties, carefully lift the clump and use a spade, sharp knife, or two garden forks placed back-to-back in the center to split it into new sections.
Remove any older, unproductive sections. Keep hold of new pieces that have several healthy shoots and roots, and replant them immediately into gaps in your borders.
Keep a close eye on when to water plants, as you need to keep the soil consistently moist to stop your asters from suffering from transplant shock and help them bed in.
5. Phlox
Phlox includes a huge range of different plant species and forms. In this scenario, the focus is on perennial border phlox, such as those tall varieties of Phlox paniculata that stand out in borders with their domed flower heads atop sturdy stems.
These stalwarts of cottage garden ideas do become less vigorous over the years. Congestion and competition mean fewer flowers, and the clump can get bare in the center. It is advisable to divide phlox every 3-4 years to keep your plants healthy and productive for the best blooms come summer.
It is beneficial to water plants ahead of dividing to keep the roots hydrated. Lift the clump and divide it into sections that have a good selection of roots and at least three shoots. These can be replanted into borders.
There is a good chance that the new clumps will flower that season. To give them the best chance, feed them with a vitamin solution such as Superthrive, available on Amazon. It can help the plants develop roots and recover from dividing.
6. Black-eyed Susan
Black-eyed Susans bring warming colors to borders in summer and early fall, with their daisy-like flowers in shades of orange and red standing out against the dark center that gives them their name.
When you grow black-eyed Susan, the plants do spread and self-seed around a border. It can lead to large clumps and congestions, with fewer or smaller flowers as a result.
Divide plants every 3-4 years in the spring to reinvigorate clumps and stop the spread, and get new plants to fill in spaces in your flower beds.
Like others on this list, dig up the clump once it starts growing and concentrate on splitting the younger, outer, more productive part of the plant into new, smaller sections.
Plant the black-eyed Susans right away elsewhere in your borders, at the same depth they were planted.
7. Ornamental Grasses
When you grow ornamental grasses among your flowering perennials, they add height, texture, and movement to borders.
Just like other plants, a substantial clump can lose vigor over the years as it spreads. Grasses can also end up with bare centers. To avoid this happening to your grasses, aim to divide them every few years.
The grasses want to be actively growing when divided. Cool-climate grasses start growing in early spring, while warm-season grasses start later but should be waking up in time for dividing in May. This month can be an ideal time for warm-season types, like miscanthus, pennisetum, and panicum.
Smaller ornamental grasses may be pulled apart by hand, but, at the other end of the scale, expect larger specimens to have tough roots to cut through.
A sharp knife or hori-hori may be needed in those cases, and you can get a Japanese hori-hori knife at Amazon.
Shop Tools for Dividing Perennials
A garden spade is ideal for digging and dividing plants. This heavy-duty border spade has a stainless steel blade and a strong hardwood handle to last many years.
You can divide perennials by placing two forks back-to-back in the clump. This fork has four strong forged-steel diamond-pointed tines and a durable D-grip handle.
This versatile garden tool has a range of uses, from planting to weeding. It can also be used for dividing thanks to its serrated blade edge that can cut through roots.
Another simple way to fill any gaps in your borders is to sow speedy annual flowers this month. This guide to fast-growing flowers to sow in May includes seven fantastic annuals that can fill your beds with vibrant color come summer.
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