To begin with, most perennials need renovating (plant division) every 3-5 years. Some such as garden mums need it every year while the really hardy ones like peonies, hosta, and daylilies rarely require renovating.
You can tell if your perennials need renovation by looking for the following symptoms.
- There are dead spots in the plant base.
- This is common on mums when they are not divided regularly; all the new growth is concentrated in the outer edges..
- Your plant did not flower very heavily this year. Iris are a good example of this; once there are too many fans in the plant, they crowd each other out and compete for nutrients – reducing the flower numbers..
- Sometimes, overcrowding will also lead to smaller flowers than normal. The symptoms all come from overcrowding of the roots and too much competition for available nutrients. As you suspect, the remedy is to divide your plants and space them out..
When can we do this work?
Spring or fall. The rule of thumb says that if it blooms in the early spring, divide in the very early spring before bloom or do it in the fall. If it blooms in summer or fall, divide it in the spring for best results (although you can do it in the fall as well)
In my experience, if you need to transplant perennials in the fall – the best time in USDA zone 4 to transplant perennials in the fall is September (but note my overall favorite time to transplant perennials is in the spring)
After September, the plants may not get enough time (6-8 weeks) to establish their roots and get settled before cold soil starts to retard their new root growth.
For every zone warmer than zone 4 you can delay the last date of transplanting/dividing by a week. So USDA 5 should be finished fall perennial work by the end of the first week in October. USDA 6 should be finished by the second week of October.
Later Than That
Plantings done later than September have reduced survival rates in my garden.
One problem for the beginning gardener is that in early September, the plants still look good and undamaged by frost and it seems a shame to cut them down to divide them.
Do it anyway. Your plant will thank you next year with better growth and flowers. Yes, they will look bedraggled as soon as you go at them with the shovels and shears but that’s the price you pay now for next year’s blooms.
Real beginner gardeners divide plants as soon as they poke their young shoots up through the ground. In this way, you can see where you’re digging and what size of offshoots you’re going to take.
Note that this will mean there’s no single best time to do spring divisions for the beginning home gardener following those timelines. As your plants start to grow, you start to divide.
Experienced gardeners dig their perennials as soon as the ground thaws (all perennials are dug then) and do divisions then. You can see “eyes” or future growth points on perennial roots then and divisions will work quite nicely at this time. The only thing is that you have to have faith that your plant is alive (and sometimes they aren’t). 😉 This is an experience kind of thing.
My advice is to dig when you see the plant starting to grow if you’re unsure of yourself. Get a bit of experience. Then start digging as soon as the ground thaws out for most of your dividing.
Or. Do it when you have the time. 🙂
Just don’t do it once the plant has started serious growing and is throwing out leaves as you may set the plant back (or kill a weak one)
There are two ways to divide perennials:
You can do it the hard way or the easy way.
Now, the hard way often recommended by gardening authors that don’t know any better, is to dig and pry the plant apart with two gardening forks turned back to back.
The easy way and the way used by virtually all commercial growers is simply to dig up the plant with a shovel and chop it up with the same shovel.
No gently prying, no careful sensibilities of plant health, just whack it in half with a sharp blade. Sometimes, with plants such as Veronica or other small clump forming plants, the plant will easily come apart in your hands after it has been dug up. Others such as Aruncus will almost need a chainsaw to get an established plant into pieces.
Let me suggest that when the plant (or plants if you are ambitious) is out of the ground, it is the perfect time to renovate the soil in the garden.
Put the plants on a tarp or in a wheelbarrow and cover them so they will not dry out.
Add several inches of compost to the garden bed by spreading it over top of the now-empty areas.
Turn these areas over to at least one shovel blade length deep – approximately eight inches in depth.
The deeper you turn over the soil, the easier a time the plant roots will have in their reestablishment.
If you are only working with one or two plants, add the compost to that planting area and turn it over as deeply as you can.
Do not be concerned about the roots from neighboring plants that are in the area; do not hesitate to cut them or disturb them as they’ll recover quickly enough on their own.
Once the bed has been dug and the newly arrived stones and persistent weed roots have both been removed from the garden, the perennials can be replanted.
Extra plants from the dividing can go into a new bed or can be given to neighbors.
Put the plants into the soil at the same depth they were at when you divided them. Hint: Iris rhizomes can be laid on the ground and the roots buried in a trench while peonies should be replanted so their eyes (the pointy things for next year’s growth) are just below the surface if you want them to bloom again. Too deep with either plant and you’ll not see blooms again until you divide them the next time.
I sometimes get asked how long the plants can be left out of the ground during the renovation? If they are kept cool (put them in the shade) and dampish (cover them with an old damp sheet or wet newspapers and spritz them with the hose once a day) they’ll keep for several weeks. As long as you do not leave them out in the sun to bake, they’ll be fine.
Do cut back the foliage on the plant by at least one half to three quarters. They do not require it to set new roots and this foliage will only lose water while the plant is waiting to be replanted.
You can read other tips about perennial flowers here.
Propagation articles can be found here