A plant needs dividing if it has outgrown its intended spot or is encroaching on other plants, if it has lost vigor and flowers have become smaller or have stopped appearing altogether, or if the middle of the plant dies while the plant continues growing in a circle around the dead center.
The general rule is to divide spring and summer blooming perennials in late summer or fall, after bloom, and fall blooming perennials in spring, before bloom.
How to divide? First, decide what to do with the extra plants produced from division. Either plant them in a new spot, in which case you should prepare that spot before you start, give divisions away to friends, or compost them.
Next, dig up the plant. Decide how many sections you want to divide it into. In general, the larger the sections, the more quickly they will recover from the shock of division. However, if you want a lot of new plants and are willing to take care of them, many small pieces will work fine. You may have to wait longer for the plants to reach blooming size if you cut the plants into small pieces.
Take a look at the root area. A trowel or your hands will easily separate loosely knit roots like those of coneflower or primula. A sharp knife is best to separate the tougher roots of plants such as peonies, phlox or astilbes. A quick downward chop with a spade may be necessary for big tough roots. A heavy-crowned plant like daylily is best separated by inserting two garden forks back to back into the center of the plant and pulling them apart.
Once you have the plant divided, carefully pull out any weeds that are entangled in the roots and cut off any diseased or rotten areas.
Remove one-half to two-thirds of the plant’s foliage so that it can concentrate on getting its roots re-established rather than supporting top growth.
Replant the divisions as soon as possible. Don’t allow them to dry in the sun or wind. It’s a good idea to throw a little compost into the hole before planting. Plant at the same level the plants had been growing and water well. If plants sink down after watering, lift them carefully to the proper level and tuck more soil under them.
A foliar feed of liquid seaweed or fish emulsion provides trace elements and speeds establishment of the plant. Do not use a regular fertilizer. It will encourage top growth which isn’t what you want right now.
Protect the plants from hot sun and wind until they are established and keep them well-watered until the ground freezes. After it does, mulch the plants heavily to prevent shallow freezing and frost heaving.
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