Dividing perennials

Dividing perennials

Early spring is an excellent time to divide and thin some perennial plants.  Plants recover from transplanting much easier in cool, damp weather.  Dividing your perennials is often necessary to keep them healthy and blooming and keeps the flower beds from becoming a tangled mess.  You can start new beds or give away the divisions.

Some species of plants need dividing more frequently than others.  A few species do better if divided in the fall.  And there are species of plants that you don’t divide unless it’s the only way to get a new start of the plant and others you shouldn’t divide at all.  While there are guidelines for the amount of time between dividing different species of plants the gardener should always use their own judgement.  In general if the plant has numerous crowns, (crowns look like individual plants) looks like it’s packed into its space and has stopped blooming well it’s probably time to divide it.  You may want to divide and thin out some species of plants if they are encroaching on other plants space.

You may not want to divide plants if they have grown little since you planted them, even though as a species it’s common to have to divide them frequently.  Each plant in its unique environment has a different growth pattern.  Keeping records, especially photo records, helps the gardener remember which plants looked overcrowded the previous season and need to be divided.

You can do the division as soon as you see new growth or even before if you know the location of the plant.  It’s easier to work with plants when the foliage is still small. Decide in advance if you are going to keep and re-plant the divisions or compost them.  Be prepared to either pot up divisions or re-plant them in another garden bed soon after dividing them, the sooner the better, if you want to keep the plants.

You’ll need a good spade and possibly a heavy duty knife, to dig and divide your perennials.   First dig around the perennial clump and lift the whole clump out on a tarp or piece of cardboard.  Gently wash away most of the soil with a garden hose if you have access.  Study the clump and decide where you can easily divide the plants and how many divisions you could make.  Usually 2-3 crowns and root sections are kept together.  Sometimes you may want to divide the plant into individual crowns and roots.  You want a good healthy looking clump for each new plant.

You can sometimes separate the plants with your fingers, gently pulling the clump apart.  Other times you may need to use a knife or even the shovel to divide the plants.  Some plants like ornamental grasses may need a saw to divide them. Some roots and foliage may be damaged when divisions are made.  This is unfortunate but necessary in certain cases.  Most perennials are pretty tough and recover quickly, especially in early spring.  In some species the center of the plant or other portions may have dead areas.  These should be cut out and discarded.

Keep your divisions moist and out of the direct sun until you re-plant them.  After planting keep them watered unless it’s a wet season so they can quickly establish new roots.  A little slow release fertilizer mixed into the soil also helps. Do not use Epsom salts on new plants.  Most divisions will bloom the same year, but smaller divisions may skip a season while they establish themselves.

Here’s a list of perennials that can be divided in early spring and notes on how often and other considerations.

Aster 1-3 years

Astilbe – 3-5 years

Beebalm (Monarda) 1-3 years

Bellflower (Campanula) 3-5 years

Blanket Flower (Gaillardia) 3-5 years

Catmint (Nepeta) 3-5 years

Chrysanthemum (hardy mums) 1-3 years

Common Sundrops (Oenothera fruticosa) 1-3 years

Coneflower (Echincea) 3-5 years

Coralbells (Heuchera) 1-3 years

Cornflower (Centaurea) 1-3 years

Cranesbill (Geranium) – may never need, divide infrequently or to get new plants

Daylily (Hemerocallis) 3-5 years

Delphinium – 1-3 years

Dianthus/ Carnation - 1-3 years

Fernleaf Bleeding Heart (Dicentra eximia)    1-3 years

Foamflower (Tiarella) 1-3 years

Gay Feather (Liatris) 3-5 years

Goatsbeard (Aruncus) after many years

Gooseneck Loosestrife, Yellow Loosestrife (Lysimachia)  3-5 years

Hosta varieties vary- some never need dividing, some are divided only when new plants are wanted, others get very crowded in a few years.

Heliopsis – more than 5 years or never

Japanese Anemone (Anemone x hybrida) 5 plus years

Lady's Mantle (Alchemilla) many years to never

Lemon Balm 1-3 years

Lungwort (Pulmonaria), 5 years plus, may take a year to recover

Mallow (Malvia) 3-5 years

Meadow Rue (Thalictrum) after 5 plus years

Meadowsweet (Filipendula) more than 5 years or never

Mints of all types 1-3 years

Obedient Plant (Physostegia) 1-3 years

Oregano- 3-5 years

Painted Daisy (Tanacetum) 1-3 years

Penstemon 1-3 years

Phlox- upright(Phlox paniculata) 3-5 years

Rudbeckia 3-5 years

Sea Thrift (Armeria) 3-5 years

Sedums, tall upright species every 3-5 years, creeping species every 1-3 years

Shasta Daisy (Leucanthemum) 1-3 years

Speedwell (Veronica) 3-5 years

Spiderwort (Tradescantia) 1-3 years

Tall Phlox (Phlox paniculata) 1-3 years

Tickseed (Coreopsis) 1-3 years

Yarrow (Achillea) 1-3 years

Divide in fall

Here are some plants that do better if divided in the late summer or fall.  Bearded iris, every 3 -4 years, Jacob's Ladder (Polemonium), every 3-4 years, Creeping phlox 3-4 years, and Siberian Iris- infrequently, as needed.

Bulbs including lilies, tulips, daffodils and so on are divided in the fall.  (Dig in fall, separate bulbs and re-plant).  Peonies can be divided in the fall but only after many years, when they have reached large clumps and it’s necessary only if you want to propagate them.

Don’t divide

Plants that shouldn’t be divided include anything with a single, woody stem.  Also these plants do not care for division : Baby's Breath (Gypsophila), Balloon Flower (Platycodon), Butterfly Weed (Asclepias), Cimicifuga, Clematis, Evening Primrose (Oenothera missourienis), False Indigo (Baptisia), Flax (Linum), Gentian, Lavender,  Lupine, Monkshood (Aconitum),roses, rosemary, Russian Sage (Perovskia), garden sage and yucca. 

If a plant suckers - produces new plants from the roots a short distance from the parent plant- those new plants can be dug in either the spring or fall and transplanted.  Some examples are lilac, trumpet vine and some older roses.  Sever the connecting roots to the parent plant and move the young plant either when the leaves are still small in the spring or after it goes dormant in the fall