I thought I would ask one of our local suppliers/growers and gurus of plants for advice on the ins and outs of properly pruning perennials this fall. As luck would have it, they said yes and here is some great information for preparing your garden for the arrival of old man winter.
One must first understand the different types of perennials.
There are 4 basic types of plants. They are: (1) die back completely to the ground, (2) those that die back to a rosette, (3) the semi evergreen and (4) the evergreen types. Remember there are always exceptions to the rules and when in doubt do a little bit of research or ask a trusted gardener what they have done in the past to have success.
First, the easiest type to cleanup is the varieties that completely die back to the ground like Hosta, Phlox paniculata ( tall phlox), Aegopodium,Alliums, Astilbe, Brunnera, Cimicufuga, Crocosmia, Convallaria (Lily of the Valley), Dicentra (bleeding hearts), Ferns that die back to the ground, if evergreen or semi-evergreen, let them be Gallium, Heucheras, Liatris,Lilium, Hemerocallis (daylilies), Hostas, Papaver(poppys), Paeonia (peonies), Physostegia, Saponaria, Tradescantia ,Tiarella, Mentha’s (mints), to name some that die back completely and thus all you have to do is shear back. If you are trying to be more of a naturalist and working with mother nature, many experts are now saying to leave the foliage on for our native pollinators/insects and garden friends like mice, moles, voles, chipmunks, etc. These wildlife like some foliage to use for protection from our cold and snow. Please note, if you choose to NOT trim your plants, some may be lost as they may be consumed for food by rodents. If you do leave the foliage on, remove only after the spring frost has left the garden. Caution must be used to shear or not to shear, I have learned some plants should be sheared back as I do not want rodents where I have planted them and if not well established may hinder survivability.
The second type that you need to clean up are those that you need to cut back to around 2-6 inches, a tight rosette. These plants like a shear back yet not as low or as hard. The following are some of the plants that like to be trimmed back in the fall: Achillea (Yarrows), Agastache,Amsonia, Aquilegia (columbines), Buddleia, Campanula (the clump forming varieties, the ground cover varieties are to be considered semi-evergreen and lightly sheared back if you want.) Spring blooming Asters, fall blooming asters leave up the stems and trim back in the early spring once you see growth of new shoots. Brunnera benefits with shear to rosette but can be considered a semi-evergreen. Baptisia cut back to couple of inches, same with Coreopsis, Chelone (turtle heads), Centaurea, Ceratostigma Plumbago, Centranthus Rubra, Chrysthanthemums (daisys), Dendranthemum,(perennial cushion mums) Leucanthemums (daisy), Dianthus (all the low growing varieties can leave the foliage on, taller ones prefer a shear back about 4-6”. Digitalis, Echinacea, Echinops, Eupatorium (Joe pyeweed), Gaillardia, Gypsophila, Heleniums, Malva’s, Monarda, Nepeta’s Platycodons (balloon flowers), Rudbeckias (black eyed-susans), Salvia’s, Trollius, Veronica also trim back to 4-6" One exception would be Penstemons, which should be trimmed mid-May like lavender, only trimming back the dead wood.
The third is the semi- evergreen these keep their foliage and can be trimmed back if you choose, the following are a list of these more common plants which you may have in your gardens Iris: Standard variety also called German/dwarf, Siberian Irises, & Japanese Irises, The Dutch Irises die back completely to the ground and thus in the first category. Semi evergreen like Achillea, Ajuga, Arabis, Armerias, Artemisia, Bergenia, Delosperma, Delphiniums (foliage will fall off, stem is kept & bent to keep from rotting), Shorter Dianthus, Gaillardia, Geraniums, Hernaria, Hypericum, Laurentia, Lobelia, Lychnis, Potentilla, Sagina’s (Irish Moss) Salvia (Sages), Scabiosa, Stokesias, Verbenas are some of the more common varieties found in many gardens, late blooming Asters, and truly hardy Mums, Violas.
The last is the evergreen and these are the plants that keep their form & structured in the winter many know Lavenders and Iberis (candytuft), Epimedium, Bergenia, Phlox subulata (creeping phlox), Helleborus (Lenten rose), Heuchera (Coral Bells), Stachys (Lambs Ears), Teucrium (germander), Penstemon, Sempervivum, Oregano, Thyme, Sedums (especially the ground cover varieties), Most people cut back the tall Spectabile varieties to a tight rosette.
Most perennial beds benefit with a half inch to an inch of compost or aged manure to help feed and nurture the plant throughout the winter. Some varieties that like a top dressing of ½” are most of the low ground growing plants under 6-9”, the taller 6-18” growing plants generally like closer to an inch of compost and a few like daisy’s which are quick growers like a little bit more. Some also add aged manures at this time instead of compost, keep it under one inch and if possible, incorporate into your soil. This application of compost or manures and the base helps to deter rodents from chewing on roots and helps support new growth for next year. Happy Gardening and hopefully this will help you in your quest to have a beautiful and healthy garden. Article written by Mary; a wonderful person devoted to plants who operates a local wholesale nursery that produces some of the best perennials on the planet to us!! Thanks for the great information, Mary!!