Spring is the perfect time to divide and plant herbaceous perennials. After 2-5 years clumps of most perennials become overcrowded and their vigour deteriorates. As a result plants produce fewer flowers, and loose some of their ornamental value.
As a general rule, spring is the best time to divide and plant summer- and autumn-flowering plants (starting with mid-June), and ornamental grasses (miscanthus, reed grass, feather grass). This is best done at the time when new shoots appear, which makes it easier to see how the clump should be divided. Make sure you divide the plant before shoots become too tall – this way damage will be kept to a minimum.
Plants, which flower in early spring (up to early June), should be divided in autumn. There are exceptions, however. Primroses, as well as most of the flowering alpine plants (creeping phlox, aubrieta, basket-of-gold, several sedums) can be divided soon after the flowering is over. This does not harm these plants in any way. Daylilies can be divided at any time throughout the growing season; the best time for bearded irises is after flowering, while peonies are usually divided in autumn.
Here are some tell-tale signs that your herbaceous perennials need rejuvenating: clump becomes congested and contaminated by perennial weeds, which are impossible to remove. the plant produces fewer flowers each year, and goes dormant earlier in the season. the middle of the clump dies down, with new shoots growing on the perimetre.
Dig out the overcrowded clump, shake off as much of soil as possible and carefully divide into several portions, each with viable roots and strong shoots. Give away smaller portions to your gardening friends, or move into nursery bed (pots) for growing on. Discard dead parts of the plant and woody roots, carefully removing any perennial weeds. If you intend to plant the rejuvenated plant back into the same spot, replace some of the planting soil. Plant at the same depths as before, and water well.
Some plants are easily divided by hand, but a sharp knife will come useful while dividing others. Old clumps of miscanthus are notoriously difficult to divide. Clumps of deadnettle, lady`s mantle, lungwort, sea thrift, creeping jenny, coralbells, columbines, bugleweed, bleeding-heart, primula, creeping phlox, maiden pinks are easily divided by hand. But you will need a sharp knife to divide overgrown plants of yarrow, bellflower, rudbeckia, coneflower, bee balm, asters, hosta, goldenrod, catmint, astrantia, salvia, bugbane, some of the poppies, aconitum and perennials tickseed.
© Giedra Bartas, 2010