Planting in winter

Our season of fall planting is nearing its conclusion. But if you need to finish off the landscape this year, or are like me and want to add “just a few more” new plants to the garden, here are some ideas and tips to ensure success with your new transplants. In fact, there are some growers who love to plant in the winter, as long as these tips are followed.
How do plants react when temperatures drop?

Most foliar growth slows or ceases in winter. Pines are one exception. As foliar growth slows, root growth speeds up. Even in deciduous species that lose all their leaves in winter, plant growth continues below the surface. And for new transplants, we actually want the most activity to occur in the root zone so that the plants can become established sooner and better able to withstand stress. Stress in winter includes frost, wind, dry soil (drought), wounding and some (limited) root damage from rodents. At any time of year, transplanting itself is a stressor to the plants and damages fine roots, which make up the bulk of root density and are the ones responsible for water and mineral uptake. Larger roots are mainly for stability and storage.
Soil is a great insulator, so instead of the twenty or thirty degrees difference between night and day air temperatures, the soil temperature changes only slightly from day to night, and is generally warmer than average air temperatures. But eventually, even root growth slows as soil temperatures lower beyond a certain point.

There are many species variation, even among California native plants, but in general, root growth is greatest when the soil temperature is in the low-to-mid 60’s Fahrenheit, which is late February through April in our low elevation gardens. Below 35 degrees Fahrenheit, almost all growth ends, but we don’t have soil temperatures that low. For instance, our Porterville CIMIS (California Irrigation Management Information System) station reports that soil temperature on December 2 was 53.6 degrees Fahrenheit, which is about as cold as it’s likely to get. This measurement is taken by a sensor six inches below the surface of a flood irrigation grass pasture; the temperature is likely to be warmer at lower depths and may be warmer or colder by a few degrees in your own garden. But it does give a general picture. CIMIS is a free public resource from the California Department of Water Resources (DWR). The Porterville station, number 169, can be accessed for both recent and historical soil, air and precipitation data. CIMIS is the system that most wireless smart irrigation controllers use. It’s not perfect, but it can give some idea of what’s going on in our climate.

In order to protect your late fall and winter transplants, cover the soil with some layer of mulch. Even a layer of old (disease and pest-free) leaves or a thin layer of wood bark is helpful. Perhaps leave the cool season weeds around the new plants if you have nothing else, for they will insulate the top layer of soil somewhat. Slopes can be hard to mulch, but as warm air rises naturally up slopes, there is some protection in that way from the coldest temperatures.
Keep the soil moist but not saturated during periods of frost warnings. Wet soil is colder than dry soil, but root growth is greater if there is some soil moisture throughout the cool season, and flowing water will melt any ice crystals. Water your plants in the morning or early in the day, after the air temperature is over 32 degrees Fahrenheit. During warmer (non-frost periods), don’t add irrigation unless more than the first few inches of soil dries out, just as we would do at all other times of the year. You can often measure the soil temperature at depths down to your wrist or so pretty easily in good soil. If the soil below the surface feels slightly warm in the early morning, those are perfect conditions for your new transplants.

Avoid fertilizing your plants or adding any amendment to the planting hole. You don’t want to encourage any foliar growth during winter. Even with frost-hardy plants, new growth can suffer frost damage. These winter stresses don’t often show their full effects until the next warm period, and sometimes not until next summer, at the first drought stress. Take care of your new transplants now for the healthiest plants next summer. If your plants seem to just sit there, not growing, but otherwise look fine, don’t worry. The plant is most likely growing roots. Which is perfect.

(This article first appeared in the Porterville Recorder on 12/5/15)