University of Vermont
Department of Plant and Soil Science

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AUTUMN PERENNIAL GARDEN QUESTIONS

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

Here are a few questions (Q) that you may have for this season if you grow herbaceous perennial flowers, and answers (A).

Q. How can I maintain a smaller rounded habit of perennials such as Sedum 'Autumn Joy' and Coreopsis 'Moonbeam'?  They are 4-5 years old.  A. Often plants too tall or floppy are a sign of too little light.  Both of these plants in particular grow best in full sun.  Even then, with age some plants, such as the Autumn Joy, may flop.  Plants that bloom late in the season such as Autumn Joy, asters, or even tall garden phlox may be cut back by one third to one half in early summer.  This will result in shorter growth with more branching, and slightly delayed bloom (which provides a longer bloom season for you and pollinators).  This cutting back generally won’t work with thin stems, as on the Moonbeam.  Make sure this one has full sun, and not too much fertilizer nor rich soil, any of which can cause tall and floppy stems.

Q. What do you suggest for a grass to plant along the driveway like a hedge?  A. If you want a tall grass (four to five feet), then consider one of the Switch grass (Panicum) cultivars.  Heavy Metal is bluish with reddish seed heads.  There are several other good blue cultivars, but Prairie Sky tends to flop.  Shenandoah is shorter, with reddish leaves.

Another group for a great upright effect of similar height is the Feather Reed Grass (Calamagrostis), Karl Foerster being a popular and good cultivar. These are being used just for this purpose all over the University of Vermont campus.  If in a warmer climate or microclimate (USDA zone 5 and warmer), you might consider Fountain Grass (Pennisetum), about three feet high. There are many good cultivars of Eulalia (Miscanthus), from four to eight feet, which with short and cooler seasons in the north do not tend to seed nor become seed invasive.

Q. What are the most common problems that I should be aware of with soils?  A. If a soil has a problem in our area, it is often too low pH (too acidic).  This can be corrected by adding lime, according to a soil test.  A soil pH that is between 6 and 7, 7 being neutral, is often best.  A soil pH that is too low or too high makes nutrients unavailable to the plant. Soil test sampling bags are available from state agriculture testing labs, often at state universities, or from garden centers.  If purchasing inexpensive soil testing kits that you can use at home, make sure they are new, as old chemicals in such kits can give wrong results.  Fall is a good time to add lime, if needed, as ground or dolomitic limestone is slow acting.  Add it now and your soil will be ready for spring planting.

Q. I have hostas with twisted, stunted, and puckered leaves.  Is this normal, or a disease?  A. It depends.  Some varieties show twisted leaves normally, but there is a virus that could be the cause on others, called Hosta Virus X or HVX.  Some varieties such as Eternal Father, Lunacy, and Leopard Frog actually have their traits due to less virulent viruses.  This HVX virus, though, causes such deformed traits, and is highly contagious through contact of infected sap from one plant with another.  This is commonly spread by hands or tools, such as through pruning, so make sure to wash in between with antiseptic soap.  As with other viruses, there are no cures, so infected plants should be discarded.   Also, like many viruses, plants may carry this one yet not show symptoms, which makes diagnosis sometimes very difficult.

The cultivar Breakdance has been reported 100 percent infected, while commonly infected cultivars include Gold Standard, Striptease, and Sum and Substance.  Before buying these, or in fact any hostas, get familiar with what they should look like, and don't buy them if they look otherwise.  It is easiest to see symptoms on gold and gold-centered plants which, in addition to these symptoms, may include random green mottling and mottling along the veins.  Since this virus must be transmitted in sap and living plants, you can safely plant where an infected plant was removed, as long as there are no living roots from the old plant.   Considered resistant are the cultivars Blue Angel, Color Glory, and Frances Williams.  Considered immune are Bressingham Blue, Frosted Jade, Love Pat, Great Expectations, Sagae, and (sieboldiana) Elegans.

Q.  Should I cut back perennials in the fall, or wait until spring?  A.  Ideally you should wait until spring, so the plants can recycle their leaves and nutrients back into the soil, provide winter cover for wildlife, and seeds in fall for birds. Leaves of some protect the plant crowns over winter. But, with this not being an ideal world, many gardeners (including me) cut them back in late fall as there is so much else that really needs attention in spring. If foliage is diseased, make sure to discard it rather than adding to the compost. 

An alternative, hybrid approach would be to leave some perennials that are attractive and favored by birds, cutting back the rest.  Coneflowers (Echinacea), Black-eyed Susan (Rudbeckia), ornamental grasses, ferns, coralbells, and sedum are perennials that you might leave and then cut back in early spring.  Bearded iris, catmint (Nepeta), bee balm, daylilies, perennial sunflowers (Helianthus), hollyhock, peonies, tall garden phlox, perennial salvia, speedwell, false indigo (Baptisia), and yarrow are among those perennials to cut back in late fall.  They either become unattractive late in the season or flop to the ground with the first snows.

Lots more lists and tips on what plants to cut back when, and other culture, can be found in the book by Tracy DiSabato-Aust, The Well-tended Perennial Garden

Return to Perry's Perennial Pages: Green Mountain Gardener Articles-- your reliable source of gardening information for over 50 years.