Hardy perennials keep dying
Question: Last year, I planted a coreopsis and a snowcap Shasta daisy, but neither grew back this year. This is the second or third time I have planted these over the past few years only to have them not come up in the spring. Any thought as to why they die and don't return?
Answer: Though it's not uncommon for the occasional perennial to die out during the winter, it's very unusual to have it happen as frequently as you have. This tells me you may be doing something wrong, either during the planting process or in the way you care for your plants, that is affecting its winter survival.
Both of the plants you mention are fully hardy here in Western Pennsylvania, so that's the first hurdle jumped. Gardeners who push the envelope and plant things that are marginally hardy in our zone shouldn't be surprised when these plants don't survive the winter. However, coreopsis and Shasta daisies should both readily survive our winters with no ill effects.
After double-checking a plant's hardiness, I then look to the planting practices of the gardener. If these perennials were planted very late in the autumn, the roots may not have had the chance to become established before winter settled in. When this happens, their root balls can “heave” up out of the soil during winter's freeze-thaw cycles. Heaving causes the roots of newly planted plants to sometimes freeze out. To ensure this doesn't happen in the future, try to have all fall-planted perennials in the ground by early October and keep them well watered until the ground freezes. You should also take a weekly walk through your garden and use your foot to gently push any heaving perennial root balls back into the soil.
A second possible cause for the death of your perennials is over mulching. This, unfortunately, happens a lot. Some gardeners think that by piling mulch over the tops of their plants, they're affording them an extra level of winter insulation when in fact they're laying the kiss of death upon them. Putting mulch of any type directly on the crown of any perennial almost always leads to crown rot, a fungal disease that causes the crown of the plant to turn to mush. When mulching newly planted perennials (or established perennials, for that matter) always keep the mulch product 3 or 4 inches away from the crown of the plant. Most hardy perennials don't need supplemental winter mulch at all, so it's a practice you can totally skip.
A third reason your Shasta daisy and coreopsis may have called it quits is because of rodent damage. In the winter, voles and mice often feed on plant crowns, roots and tree bark because other food is more scarce. If you have a population of either of these critters living in your garden, they could be to blame. I sometimes find the voles have nibbled my daylily tubers and iris rhizomes after a long, hard winter. Both of these animals feed at night, so gardeners don't always know they're even in the garden. Round, quarter-sized holes in the soil (entrances to their burrows), coupled with nibbled plants and missing tulip or crocus bulbs could point to them as a possible culprit.
Good cultural practices can also help ensure the return of your perennials every year. Make sure the plants are well-watered throughout the growing season and into the fall, avoid cutting them back until the spring, and don't fertilize late in the growing season. All of these smart gardening practices should help your plants thrive for many years to come.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com.
Send your gardening or landscaping questions to firstname.lastname@example.org or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.