To prune or not to prune your perennials is the question that arises about this time of year when many have stopped blooming and your garden is beginning to look a bit unkempt. You have, of course, been dead heading during the bloom but now might be the time for a more drastic measure. I often have to consult a reference book to see if the perennial should or shouldn't be pruned, so assuming others have this same problem and would like an easy reference, I did a little research. Unfortunately, this article cannot list every perennial you might have in the garden so I will list the most common for this area. If you should have a question regarding any particular perennial, please call the Extension Office with the question and we will find the answer.
The first name in the list is the species while the name in brackets is the common name.
ACHILLEA (Yarrow)- when the bloom is completed, shear down to basal foliage. If your yarrow is being aggressive, cut back foliage as well as roots. Leave the basal foliage over winter.
AEGOPODIUM PODAGRARIA 'VARIEGATUM' (variegated bishop's weed) – mine was beautiful this year with all the rain and the cooler weather but as soon as the heat arrived the leaves started to turn brown. Whatever the reason – cut, shear or mow to the ground for a flush new growth – and it will return quickly. Very invasive so might have to pull out some plants.
AJUGA (bugleweed)-deadhead to prevent excessive seeding, then shear to prune. Occasionally can mow but not repeatedly. Thinning plants reduces crown rot. If infected, prune out diseased sections. These plants are evergreen so do NOT prune for winter.
ALCEA ROSEA (hollyhock) Cut back to new basal foliage as soon as all the flowering is completed.
ANEMONE – deadheading does not prolong bloom. Plants can be cut back if they blacken after a hard frost.
ARTEMISIA LUDOVICIANA (silver king artemisia) – plants should have been sheared back by ½ at the end of May to control flopping and the same done in mid July – if they are falling over, prune them back. Very invasive and the spread can only be controlled by digging out the invasive roots.
ASCLEPIAS TUBEROSA (butterfly weed)-deadheading results in re-bloom about a month after initial flowering. Allow some of the second bloom to mature into the ornamental fruit for interest. It is a heavy seeder so remove the fruit before it splits. Do not prune for winter, cut back in the spring.
ASTILBE - Deadheading will not induce more blooms and the dried seed heads extend the interest through the season and even into the winter. Do not cut back for winter, but prune in early spring.
AURINIA (basket-of-gold) – plants should have been sheared back by 1/3 after flowering. Plants may require dead leafing of rotten and tatty leaves during the summer months to keep them looking fresh. Do not cut back for the winter as this species is evergreen or semi-evergreen.
BASTISIA AUSTRALIS (blue wild indigo)- Cut plants back 1/3 after flowering and they will hold up well for the rest of the season (remember that next spring after they bloom). Cutting back helps to keep the plants in their intended space. Unfortunately, this eliminates the seed pods that are interesting in the winter – but you can compromise and cut back only part of the plant. You will probably want to cut the plant down after several killing frosts as it will turn black.
BELAMCANDA CHINENSIS (blackberry lily) – plants will reseed, sometimes heavily, but allowing some seeding is recommended to ensure the presence of this often short-lived perennial. Remove any dead leaves and clean-up debris at the base of the plant during the summer.
CAMPANULA (bellflower)-deadhead to lateral buds to prolong bloom. As foliage starts to decline and flowering is completed, cut down to fresh basal foliage. Do not prune for the winter.
CLEMATIS (ALL TYPES) –could write an entire article on this perennial. Best to deadhead if possible but do not prune now.
COREOPSIS GRANDIFLORA (tickseed)- daily deadheading. Prune to basal foliage at the end of August or early September. Do not prune for winter.
COREOPSIS VERTICILLATA (threadleaf coreopsis)- plants can be sheared in August to remove deadheads. Do not prune again until early spring unless seeding is a problem, then cut down in autumn.
CHRYSANTHEMUM (mum) – the pruning of mums should have been done before July. Do not prune after they flower as this will improve their overwintering survival rate.
DIANTHUS BARBATUS (sweet william) you can cut back by 1/3 to ½ immediately after flowering and before seed sets to get this biennial to act perennial or you can let it go to seed and enjoy the progeny. It self-sows so easily it seems to be a perpetual in the garden. Do not prune for winter.
DIANTHUS GRATIANOPOLITANUS (bath's pink cheddar pink) – cut or shear off old flowering stems and about 1/3 of the foliage after flowering – which was back in the spring. Basically should remain a dense ground cover for the rest of the season, including into winter.
ECHINACEA PURPUREA (purple coneflower – or any coneflower) – this is a plant that doesn't need to be deadheaded, but I remove the blackest heads and allow the other seed heads to remain for the birds. However, this can mean excessive seedlings in the spring. Some of your coneflowers can be pruned BEFORE flowering by cutting back by ½ in early June (or when 2 ½ feet tall) – and this will allow this group to flower until late September while the unpruned plants will have stopped blooming in August.
GAILLARDIA (blanket flower) – Cutting all flowering stems down to the basal foliage in late August or early September can stimulate vegetative growth before the coming of frost.
GERANIUM (hardy geranium)-plants hold up well until late winter, at which point they often turn to mush. Cutting back in the autumn after several killing frosts may be desirable.
HEMEROCALLIS (daylily)- First, when all flowering is finished on the flowering stem the entire stem should be cut back to the basal foliage. This not only keeps the plants looking good but is very important for the repeat in repeat-blooming daylilies. After all flowering and deadheading is finished, around mid-August, dead leafing begins. This involves grabbing clumps of dead leaves and pulling them out of the plant by hand. If foliage decline is severe, it might be more practical to simply shear the whole plant down. New foliage will emerge and hold through to frost, but be sure to keep the plants moist after shearing.
HEUCHERA (coralbell)-Plants might need dead leafing if they get too dry. Flower stems can be trimmed off to the basal foliage, but basically the evergreen foliage usually holds well over winter.
HOSTA (plantain-lily) – deadheading improves appearance, but leaving foliage on hostas over the winter provides added protection.
IRIS (bearded iris)—when all flowering is finished, the flower stalk is cut off down to the foliage. If plants aren't diseased, selectively pulling off and trimming the worst leaves is fine. If badly affected, shear all foliage back to about 4-6 inches above the ground.
IRIS SIBIRICA (Siberian iris) Wait until spring to cut the entire plant down.
LAVANDULA (English lavender) Avoid heavy pruning after August so plants are able to harden before winter.
LEUCANTHEMUM (shasta daisy)- deadheading prolongs bloom. They continue to bloom, but if you cut the plants down in early September, vegetative growth is stimulated and will prolong the life of the plant.
MONARDA (beebalm)-after flowering, the foliage may be infected with mildew so it is best to cut it back to the clean foliage developing at the base of the plant
PAEONIA HYBRIDS (peony) – it is best for the health of the plant to leave the foliage on the plant as long as possible. Plants not cut back in the late summer or autumn should be cut down for the winter to remove possible sources of infection.
PHLOX PANICULATA (border phlox) be sure to prune down phlox for the winter if they are affected with mildew.
PHLOX STOLONIFERA (creeping phlox)—cut or shear deadheads back to the foliage after the blooming is finished. Plants remain evergreen over the winter so do not prune in the autumn.
RUDBECKIA (black-eyed susan)- long bloom period even without deadheading. Stems hold up well and the basal foliage remains evergreen.
Sorry, will have to stop here as way over the word limit now even if I didn't come to the end of the list.
If you have any horticulture questions, call the U of I Extension office 345-7034. Volunteer
Master Gardeners are in the office on: Monday—2 to 4 p.m.; Wednesday and Friday—9-11 a.m.
This column is based on information and materials at the University of Illinois Extension office, located at 707 Windsor Road, Suite A., Charleston, 61920; phone 345-7034; or web site: www.extension.uiuc.edu/coles/