Dividing perennials not only gives you more plants, it also maintains the health and vigor of the plants.
Of course, it depends somewhat on the perennial. Many perennials, such as irises, daylilies, tall phlox, creeping phlox, daisies, blanket flower, coreopsis, mums and yarrow will bloom less if they are not divided every few years. However, a few perennials, such as peonies, balloon flower, monkshood and butterfly milkweed are perfectly happy with no division at all. And a few perennials should never be divided, such as perennial types of poppies, which have a deep taproot that doesn’t transplant well.
What perennials to divide?
If you’re unsure about whether or not to divide a perennial, keep an eye on it throughout the year. The four reasons to divide perennials are if:
• They have stopped blooming as heavily as they once did
• The clump has turned into more of a ring, with a dead spot in the middle or
• The plant has spread rather willynilly and you’d like to tidy up the sprawl into one, more compact area or
• You’d like more of that particular plant for yourself or to share with others.
If you’re still unsure, you can do a bit of an experiment to find out if your perennial will respond well to division. Usually, when you divide plants, you dig up the entire root ball and then break or cut it into sections, which you then replant. If you’re worried about the impact of digging up the entire established plant, just take your spade and dig up a slice or corner of the roots, leaving most of the root ball undisturbed. If that root division does well in its new home, you know that perennial will transplant well.
When to divide
Timing is important. As a rule of thumb, you should not divide a perennial for several weeks before it’s due to bloom. That’s because flowering requires a lot of energy from plants, and if you transplant it right before or during bloom time, the plant can’t divert that energy into developing good, strong roots—critical for long-term success.
That means spring is the time to divide perennials that bloom late summer and fall. And fall is the time to divide spring-blooming perennials. (Avoid dividing nearly anything in the hot, dry weather of late July through early September. Weather conditions make it just too stressing for most plants to establish themselves well.)
If you must divide a plant when it’s in flower, trim off all flowers to divert that energy into root development.
It’s ideal if you can divide a perennial when it’s just a few inches high, which in Iowa means early spring. Shorter, smaller plants adapt to new conditions more easily, because their damaged root systems don’t have to support so much stem and foliage.
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For this reason, if you find you have to transplant something with fully developed foliage — a common problem in the fall — after transplanting, trim off everything to just a few inches above the ground.
How to divide
Start with a plant that is well hydrated, either right after a rain or a day or two after you’ve given it a good watering. This helps it survive the stress of transplanting.
If you get to choose what day you’re transplanting, the ideal day would be cool and overcast. If rain is forecast soon, so much the better. Transplants don’t take as well in bright, hot sun and dry conditions.
Then dig up as much of the plant, roots and all, that you can. Some plants, like mums and hardy geraniums have shallow root systems that are easy to dig with just a trowel. But those with big, compact root balls, like Siberian irises, daylilies, and peonies, will require some time, muscle, and a strong spade.
Some perennials will break naturally into sections as you dig them or can be easily pulled apart with your hands. But those with the big, compact root balls require more aggressive techniques. You may need to use the spade to slice it into sections. If that doesn’t work, a small hand ax can be used to hack it into sections.
Improve the soil wherever you are going to plant the new sections. This will be one of your few chances to improve the soil texture directly underneath the plant, where critical draining happens. Work in a few spadesful of compost, which not only improves the soil texture but also soil fertility and attracts beneficial earthworms and microbial activity.
Dig down, if possible, at least several inches deeper than needed for the planting hole. This breaks up the soil for better drainage and plant health.
Replant, positioning the plant at the same depth it was originally. Fill the hole, tamping down firmly with your foot. If you can, make a small moat around the center of the plant to retain water.
Then water thoroughly, soaking several inches down and eliminating any air pockets.
Keep the transplant well-watered for the next two weeks. You might also give it additional water during any especially dry spells.
After that, it will be well positioned to thrive and bloom beautifully for years to come.
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• Veronica Lorson Fowler is co-publisher of The Iowa Gardener website at www.theiowagardener.com.