By Linda Naeve
Iowa State University Extension
Nothing appears as dead and barren as soil in an Iowa farm field in mid-winter that is covered with a dusting of snow. Perennial plants in gardens have gone dormant and annuals have been removed or tilled under exposing the dark, seemingly “lifeless” earth.
Although the surface of the soil may be frozen this time of year, there is plenty of life underneath. Rather than retreat to warmer climates and environments in the winter like some people and birds do, many insects and other organisms spend the winter in the cold soil.
Unfortunately, some of the insects that attacked your vegetable plants last summer may be taking a long winter’s nap in your garden. Tomato hornworms and squash vine borers are two pests that stay around during the winter months. After feeding on tomato plants, tomato hornworm caterpillars burrow several inches into the soil where they create a small chamber, or cell, and spend the winter as pupae. The following spring, when the soil warms up and days get longer, they emerge as adults.
Squash vine borers make themselves comfortable in your garden soil when, as larvae, they eat their way out of the stems of squash and pumpkins in late summer. They burrow into the soil and are camouflaged when they pupate into a dark brown cocoons.
Some disease-causing organisms, such as the fungi that cause tomato blights and anthracnose on melons and cucumbers, and the bacterium responsible for black rot on cabbage and broccoli, overwinter on plant debris left on the soil surface.
Knowing what insect pests and disease-causing organisms overwinter in or on the soil is important for developing an effective pest management strategy. Fall tilling can bring overwintering insects closer to the surface where they may be killed by freezing temperatures. Deep tilling crop residue into the soil immediately after harvest can also effectively reduce the inoculum level in the soil.
It’s hard to believe that any herbaceous plant can possibly survive our harsh winter conditions and frozen soil. Although not visible to us, some plants continue to grow and develop throughout the winter. Tulip bulbs, for example, are planted in early fall so that they develop a strong root system as the soil temperature drops. These bulbs also use the long winter’s rest to initiate flowers. They require at least 15 weeks of cool temperatures, below 45 degrees, for proper flower bud development. Late planting and a shorter cold treatment can result in weak, poor quality blooms.
There are thousands of beneficial organisms that survive the winter in the soil. Many kinds of earthworms, such as night crawlers, tunnel deep into the soil before it freezes. They "migrate" downward below the frost line. In their deep, dark burrows, worms coil into slime-coated balls and wait for the frost to leave.
The condition of your soil and its moisture level in the fall and throughout the winter affects how well perennial garden plants will survive the winter. Dry or water-logged soils will stress and weaken plants at a time when they need strength to survive the winter.
The addition of organic matter, such as compost or manure, to the soil in the spring, prior to planting and after the last harvest in the fall will improve the health of garden soil and improve its water-holding capacity and ability to drain. Soil high in organic matter content is alive with an abundance of beneficial organisms that aid in decomposition, increase nutrient availability to the plants, and control the population of harmful organisms. Although this activity slows down during the winter, it will resume at full speed when the soil warms up in the spring.