When great care is given to your herb plants, many of them can survive in the garden for many years. Others are more sensitive to severe cold weather or frost and must be brought indoors for protection, and then replanted each growing season. Annual herbs will be killed with the first frost of the fall season. Remove these dead plants to minimize infestations of wintering insects and problems with disease. Some herbs that are sensitive to frost damage, such as basil and geranium, can be brought indoors for the winter season.
Many perennial herbs are hardy enough to withstand the winter and can be left in the garden. A few plants are only marginally winter hardy at best - they may survive a mild winter but die during a severe winter. Such plants can be brought indoors to overwinter. They may drop their leaves if they do not receive enough light. Lemon verbena, being a deciduous plant, will lose all of its leaves when placed indoors.
After a particularly harsh winter, some outdoor plants such as sage, thyme, and rue may actually appear brown and dead. It may be that their leaves are simply dehydrated or the plant may be dead almost to the ground. You can scrape the bark of a few stems to determine the extent of damage and chances of the plant surviving. If the stem is green where you scrape it, delay pruning the plant until new growth begins again.
A 2 to 3 inch layer of organic mulch - pine straw, cocoa bean hulls, hardwood bark, bark and sawdust mixture - during the growing season will benefit overwintering herbs. Mulch provides good winter protection for herbs such as chives, mint, and fennel to temperatures of -20 degrees F. A winter mulch helps to maintain a stable soil temperature around the root network and protects against heaving caused by freezing and thawing of the soil.
Some herbs need to have a thicker layer of mulch placed around their roots to protect them during an extended or particularly harsh winter. Heavy mulching before the cold weather sets in is discouraged as this will keep the soil at a warmer temperature and may actually decrease the plants winter survivability. After the first hard freeze, apply 3 to 6 inches of an organic media such as straw, pine needles, or chopped leaves. Try and use a mulch that stays loosely packed, since a packed-down mulch will tend to stay too wet during the winter and cause the plants to rot. Most of this winter mulch should be removed in the spring when new growth is seen.
Some perennial herbs, rosemary and lemon verbena for example, are not generally winter hardy. You can provide these plants with extra winter protection by cutting the plants back to within a couple of inches from the ground after the first hard frost. Then cover the remaining stub with soil and cover the soil with a 4 to 5 inch layer of mulch. A 95% survival rate for lemon verbena can be achieved through the use of a microfoam ground cover ( the same packing material used to protect fragile items) held down with soil. Another method is to surround the plant with a cage of chicken wire. The diameter of the cage should be about 12 inches bigger than the plant. Then fill the chicken wire cage with mulch.
Harsh, drying winds can be as deadly as cold temperatures to some of the plants that are less tolerant of cold. The use of wind breaks can help your plants to survive these harsh winds. They can also maintain the appearance of such herbs as French tarragon, winter savory, Roman chamomile, and English lavender. Covering your plants with some boughs of evergreen will prevent the drying out of silver and lemon thyme. The herbs that are more cold sensitive have a better chance to survive if grown with some sort of wind protection.
You should refrain from fertilizing your herb plants after early August. A late summer application of a nitrogen fertilizer will promote new growth that may not have time to mature before the first hard frost. This will also lead to the herbs continuing to actively grow before becoming acclimated to cold weather.</p>
Any significant pruning, other than perhaps a light harvesting, should be avoided in August. Such late summer pruning will tend to stimulate new growth that will not have enough time to mature before the first hard frost. Avoid heavy pruning even in the late fall as this will create cuts that may not have time to heal before winter sets in and therefore will reduce your plants winter hardiness. Woody plants should not be heavily pruned within 4 to 6 weeks of the onset of the first severe freeze. A light pruning after the first frost is acceptable.
Soils with excessive moisture or herbs planted in areas with standing water can lessen the winter survivability of some plants. Plants that are adapted to dry climates, such as the plants of Mediterranean origin that include rosemary, thyme, lavender, and tarragon, are especially hard hit by excessive moisture. Provide better drainage by surrounding the plants with pine bark mulch or planting them in raised beds.
Plants need to be kept well watered during the late summer and fall. Plants weakened by drought are less able to withstand the harshness of winter. Water your plants during a dry winter, especially before a severe freeze takes place. This is especially important with evergreen plants that lose water from their foliage on bright, sunny days even in frozen soil.