Q: Every spring I lose several herbs that are supposed to be winter hardy. Sometimes sage and lavender plants make it until spring and then collapse, and I’ve watched this happen with tarragon, too. Other plants growing a few feet away will look just fine. What can I do to improve the winter survival of these herbs?
A: When hardy herbs fail to emerge from their winter rest, or begin to grow and then wilt to death, the cause is usually some type of root rot. Numerous types of soil-borne fungi can enter plant roots and destroy them, but several easy steps prevent these problems.
Take phytophthora (fi-top’-thor-a), for example. This common troublemaker’s name is Greek for “plant destroyer,” and various species of phytophthora fungus have played important roles in human history. One strain led to the Irish potato famine of 1845 to 1847, and another is the reason azaleas and rhododendrons sometimes wither to brown. At least two species infect sage and lavender, and you could probably blame the loss of tarragon on phytophthora as well.
Fortunately, it’s fungi against fungi down in the soil, so enriching the soil with a diversified range of microorganisms — which you can do by digging in compost before planting or using it to mulch established plants — often suppresses phytophthora. In soil that is regularly amended with compost, beneficial fungi and bacteria out-compete troublemakers in the struggle for moisture and nutrients.
Buy compost in bags or make your own. Either way, the important thing is to use compost that has cured for several weeks. The final stage of decomposition is an important one, because many of the most beneficial strains of fungi and bacteria emerge as compost cures.
Even if you enrich your soil with compost, problems still can arise if the plants’ roots are forced to sit in waterlogged soil, which weakens the roots and favors many types of root-rotting fungi. The superior drainage provided by raised beds can make a huge difference, but in heavy clay soils herbs may need even more help. As you plant perennial herbs, try placing a handful or two of small stones or gravel under them, about 12 inches below the surface. This technique creates a drainage pocket for each individual plant, and it can make the difference between life and death for lavender and tarragon, which must have perfect drainage.
Beyond keeping soil teeming with beneficial life forms and doing what you can to improve drainage, it can help to propagate a few new plants each spring, so you always have vigorous 1-year-old plants going in- to winter. Youthful plants often can better defend themselves from root-rot diseases than older ones. In June, plan to root stems in seed starting mix or bend stems over and cover them with damp soil to grow new plants using the layering method.
Do give older specimens a chance to stand through winter, because 2- and 3-year-old plants are usually the strongest bloomers. A little pruning can improve the looks of your established herbs during their winter rest, but don’t prune them back to the ground. Allowing stiff-stemmed rosemary, sage and lavender to retain umbrellas of brittle branches helps to shelter shallow roots from damage caused by ice and cold winds.
Even if you do everything right, winter may claim a few casualties in your herb garden. Pull out plants as soon as you see them wilting, because herbs seldom recover once they are infected by a root rot disease. Restore the health of the soil where the sick plant grew by mixing in a quart or so of compost, and then replace it with an unrelated plant. In most cases, the same fungi that caused one herb to wilt are unable to attack the roots of a very different type of plant.
Barbara Pleasant is a contributing editor to The Herb Companion and author of several books about gardening, including The Whole Herb (Square One, 2004).