Tips for cutting back herbaceous perennials

As winter progresses, the foliage on most herbaceous perennial plants is dying back. Many are going dormant. We want our gardens to look well-kept even in winter. But what do we cut back now, what is the best way to trim and what should we save for later?

An herbaceous perennial is a plant with non-woody top growth that dies down annually, but has roots that remain living and regenerate in the spring. This plant type often has a long, showy bloom season, making them worthwhile additions to a garden.

If they are cut back neatly in winter, the repetition of their low green forms can lend the winter landscape a simple, restful beauty of its own. The old growth must be removed each year either because it is completely dead, or it is senescent (over-mature) and needs removal to let new, vigorous growth resume in the spring.

These are different types of herbaceous perennials and each type requires a different approach to fall cleanup. Most indicate the treatment they require by a few different factors detailed below:

Herbaceous perennials that die back completely to the ground: This group includes plants like perennial sunflowers, the shade-loving aster, Aster divaricatus, Baptisia, milkweeds, Salvia gauranitica and others. At this time of year the spent stems are completely dead and there is no indication that a living and vigorous plant is underground. The old stems should be cut to the ground, or an inch above it. The trick to neatness is to make sure to cut the stems evenly and don’t leave scraggly bits here and there.

Herbaceous perennials that die back to a cluster of fresh, new leaves: The old plant stems on this group are either almost completely dead or really scraggly. The old stems should be cut down as far as you can easily reach into the new, fresh growth. It is important to remove these old stems, so new growth can come from the ground, not from the base of the old stems. Growth from the old stems lacks vigor. Again, for a tidy look cut the old stems evenly and don’t leave scraggly stems. Some examples are many asters, Thalictrum, Tuecrium hircanicum, Knautia macedonica, Globe thistle (Echinops), Sea Holly (Eryngium), fennel, goldenrod, black-eyed Susans, catmint, Helenium, Campanula, lady’s mantle, Geranium ‘Rozzanne’, yarrows and more. In some plants the fresh new growth may be hiding under the old spent stems and may not be apparent until you pick them up. Examples are Knuatia, ornamental oreganos, Ballota, Lepechinia hastata, coreopsis, red-hot pokers, Mexican sage, gumplant and California fuchsias. The danger of waiting until spring to cut these back is that the new, fresh growth may be smothered by old and rotting stems. Our winter temperatures are generally not cold enough to freeze these plant’s new foliage. The exceptions may be some of the tender salvias like Salvia ‘Waverly’ and Salvia ‘Phyllis Fancy’ and some of the cigar plants (Cupheas). You may not want to cut these back until March or early April in the colder parts of our area.

Perennials that need the old stems cut back to 2-4 inches above the ground: These plants generally have new growth emerging from the lower portions of old stems, not the ground. If you look closely, you will see fresh, new leaves coming from the bottom 2-6 inches of the stems. Cut back into this fresh new growth to rejuvenate the plant. New growth mostly comes from the base of these old stems rather then from the roots. Examples are plants like many penstemons, Salvia microphylla, S. ‘Mystic Spires’ and Verbena bonariensis. Our native penstemon, Penstemon heterophyllus, needs the old growth sheared back to about 4 to 6 inches depending on the variety.

Semi-evergreen perennials: These plants are generally green year-round, but may have old leaves around the outer edges of the plant that need removal. They are easy to spot in the winter garden because they are handsome at this time. These are plants like coral bells (Huechera), Christmas roses (Helleborus), Geum, hardy groundcover geraniums such as Geranium x cantabriense “Biokovo,” Geranium macrorrhizum ‘Ingwerens’ and G. ‘Beven’s Variety, the seaside daisy (Erigeron glaucus). The old outside leaves on plants like Geum and the Christmas rose are easy to identify as they are untidy and brownish, while the new growth is green. These old stems/leaves should be cut to the ground early in winter so the plant can look its best.

I’ll write more about winter cleanup in my next column.

More information on fall cleanup will appear next column.

Dana Lozano and Gwen Kilchherr are garden consultants. Send your gardening questions to The Garden Doctors at pdgardendoctor@gmail.com. The Garden Doctors can answer questions only through their column, which appears twice a month in the newspaper and online at pressdemocrat.com.