Cut Back Perennials
So, when do you cut back perennials?
You cut back some perennials that bloom in very early spring immediately after they bloom. For example, if you cut back a lungwort (Pulmonaria) right after if finishes blooming, the new leaves will be clean and will be mildew free longer than if you left them.
A rule of thumb is to allow spring bloomers to flower and leaf out freely. But if the plant tends to get diseased early in the summer (like lungwort) then cutting it back immediately after blooming will often help this.
You can cut back perennials hard
You can cut back perennials hard (to the ground or by half) in early spring if you need to for some reason.
They will regrow those leaves if the plant is basically healthy. The nursery industry does this all the time. If a plant gets too leggy in the pot, it will be whacked back and forced to bush out and regrow new leaves to make it a presentable plant. Just understand that some plants such as Hosta do not like to be cut back and will sulk if you do.
Understand too that if you cut back perennials hard, it is a good idea to throw some compost on the plants to help them recover from the shock.
So the rule of thumb is to try to avoid cutting back really hard unless you have to for some reason. (like you want to move the entire garden).
Late spring pruning
If you have a late summer or fall blooming plant then late spring pruning has another effect.
Note that cutting back a vigorous growing perennial will thicken it up and force it to throw several shoots. For example, coneflower is noted for trying to throw a single shoot particularly in its first or second year of growth. If you allow this plant to get to 12-inches tall and then cut it back to 6-inches, youll make the plant throw multiple shoots and increase the flowering. This is particularly true of the daisy family it is mandatory to cut back perennial fall mums if you want a great show.
Will cutting back influence flowering?
If I cut back perennials in my garden, will this delay or influence the flowering?
Not with late summer or fall blooming plants, they catch up with bloom time. But for early or mid-summer bloomers - for sure. If the plant is growing new leaves, it will not be producing flowers (or not very many of them).
After flowering
What about after flowering?
Cutting back the flowers is a great idea we call that deadheading. Cutting to the ground is not a good idea unless it is very early spring. In general, once the plant has flowered, wed like to clean it up remove spent flowers and dead leaves but let it recover and put energy into those roots for next year.
What about cutting back in the fall?
Well, yes this is a good idea. Once youve had a few hard frosts or the leaves on the trees start to turn colours, you can be sure the perennials are ready to go dormant. I cut my perennials back in late September when I propagate them.
What if the plants dont look ratty or ragged in the fall?
Well, then its up to you. I do my basic fall division in late September but if plants are still looking good, I tend to leave them alone until a really hard frost knocks them ugly. Then Ill cut them back (usually all at once as I do the garden up for winter).
But yes, do cut back perennials in the fall.
Are there some you don't cut back in the fall?
Yes. I don't cut back plants that are evergreen such as Hellebore (although they often die to the ground in the spring anyway and require a pruning after flowering)
I also don't cut back perennials such as lavender or dianthus that don't die to the ground over the winter but bloom on old growth.
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