A reader writes: I'm concerned that the lack of snow cover this winter is going to be hard on my perennials. Last fall I put straw mulch on a couple of perennials that I know are marginally hardy. Should I go out and put straw on everything or is it too late?
The lack of snow cover in the metro area this winter may put many perennials and even trees and shrubs to the test.
Snow is an excellent insulator. It traps air between the snowflakes and holds in the heat of the earth while deflecting bitter cold air temperatures. Gardeners in northern Iowa tell me that they often have winter kill on some perennials that we can grow with no problem in central Minnesota. The difference? Here we usually have a solid snow cover through the coldest months of winter, while in warmer Iowa, the snow cover tends to be more spotty.
Snow or mulch -- such as straw or bags of dried leaves -- helps plants in several ways, but it doesn't prevent the ground from freezing. In fact, later in winter it helps keeps the ground frozen, though not as deeply. And that's a good thing. Here's why: When soils go through repeated freezing and thawing cycles, plant roots can readily be broken, and plant crowns can be pushed out of the soil where they may freeze or dry out. That's why we apply winter mulch when the ground starts to freeze and remove it gradually in the spring.
And while neither snow or mulch will keep the ground from freezing, it will moderate the soil temperatures and keep the frost depth much shallower than on exposed soil. By moderating soil temperature, mulches may keep plant roots and crowns from reaching a lethal low temperature.
Mulches also can hold moisture in the soil. Bare soil allows moisture to escape, even in the winter. Winter drought can lead to serious plant damage.
It's hard to tell how much injury plants already may have sustained this winter. Many plants will survive without snow cover, but I suspect we may see greater levels of injury to many herbaceous perennials and some woody plants. If you have mulch available, spread a thick layer over plants you are concerned about. At the very least, this should prevent damage from the late-winter freeze/thaw cycles.
Branches from my Christmas cactus often drop off at this time of year. What causes this?
I can't name one specific cause, but let's go through some basic information about the plant's origins, care requirements and growth habits. This may help explain why your plant is having problems.
Many holiday cacti sold by greenhouses and florists are Schlumbergera truncata, commonly known as Thanksgiving cactus. This species typically blooms heavily in late fall or early winter. True Christmas cactus is a hybrid between S. truncata and another Schlumbergera species. This hybrid blooms about a month to a month and a half after Thanksgiving cactus. There are also hybrids between Thanksgiving and Christmas cactus that have an intermediate bloom time.
Though these are true members of the cactus family, they are not at all like the cacti we might see in the deserts of the Southwestern United States. Holiday cacti are native to Brazil, where they grow as epiphytes (pronounced ep-uh-fights) in tropical trees. Epiphytes use the host tree only for physical support. Water is provided from rainfall, and nutrients come from bits of plant debris.
That means the roots of holiday cacti need regular moisture and excellent drainage. A soil-less potting mix that has extra perlite added to ensure good drainage is essential. When you water a holiday cacti, be sure to dump any extra water out of the saucer. During the short days of winter, when plants are not actively growing, holiday cacti need less water. But that amount should be increased as active growth resumes in late winter.
If the branches that drop off of your cactus turn yellow first, you may be overwatering. On the other hand, if the branch segments look withered before dropping, you may be underwatering.
Though generally tough houseplants, holiday cacti are susceptible to several diseases that can cause stem or root rots. A phytophthora fungus can produce lesions with reddish borders on the stem and lead to greyish discoloration and dropping of branch segments. Another fungus results in fuzzy black lesions on stem segments; infected segments often drop off. Severely infected plants should be discarded. Avoiding overwatering can help prevent these diseases from appearing.
The unique structure of holiday cacti also must be taken into account. Lacking true leaves, these plants actually consist of flattened stem segments that grow as arching branches. The joints between the stem sections are flexible, and the sections can be separated easily with a quick twist and pull. These stem segments can be rooted easily. While sections don't typically just fall off of healthy plants, a rambunctious pet or child could be bumping into the plant and breaking branches.
Nancy Rose is a horticulturist with the University of Minnesota Extension Service. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions in this column only.