Winterizing the Herb Garden

1/99 HIL-8112

Linda Blue, Extension Agent, Buncombe County Center
Erv Evans, Extension Horticultural Associate
Jeanine Davis, Extension Horticultural Specialist
Department of Horticultural Science
 

If treated properly, many herb plants will survive in the garden for a number of years. Others are sensitive to frost or severe cold weather and must be brought indoors, protected, or replanted each year. Annual herbs will be killed with the first hard frost in the fall. Remove dead plants in order to minimize overwintering insects and disease problems. Some frost sensitive herbs, such as basil and geranium, can be brought indoors for the winter. Take cuttings to root or pot the entire plant.

Many perennial herbs are winter hardy in all or parts of North Carolina and can be left in the garden. A few plants are marginally winter hardy; in a mild winter they survive but may die during a severe winter. They can be brought indoors to overwinter. Unless they receive adequate light indoors they may drop some of their leaves. Lemon verbena is a deciduous plant; it will lose all of its leaves indoors.

After a severe winter, some outdoor plants such as rue, sage, thyme, and southernwood, may appear brown and dead. The leaves may simply be dehydrated or the plant may be dead almost to the ground. Scrape the bark of a few stems to determine the extent of damage. If the stem is green, delay pruning until after new growth begins. Additional information on winter hardiness of specific herbs can be found in Table 1.

Table 1. Hardiness and winter care of select herbs.

Common Name

Scientific Name

Hardiness*

Comments

Angelica

Angelica archangelica

M, P, C

.

Artemisia

Artemisia spp.

M, P, C

Requires good drainage

Basil

Ocimum basilicum

--

Tender annual

Bee balm

Monarda spp.

M, P, C

.

Caraway

Carum carvi

M, P, C

Biennial; lightly mulch

Cardamon

Elettaria cardamomum

---

Tender annual

Chamomile

Chamaemelum nobile

M, P, C

Lightly mulch; provide wind protection

Chives

Allium schoemoprasum

M, P, C

Lightly mulch

Comfrey

Symphytum uplandicum

M, P, C

Lightly mulch

Coriander (cilantro)

Coriandrum sativum

---

Tender annual

Costmary

Chrysanthemum balsamita

M, P, C

Lightly mulch

Dill

Anethum graveolens

---

Tender annual

Echinacea

Echinacea purpurea

M, P, C

..

Fennel

Foeniculum vulgare

M, P, C

.

Feverfew

Chrysanthemum parthenium

M, P, C

Lightly mulch

Garlic

Allium sativu

M, P,C

Plant cloves in fall; lightly mulch

Garlic chives

Allium tuberosum

M, P, C

.

Garlic, elephant

Allium ampeloprasum

M, P, C

Plant cloves in fall; lightly mulch

Geranium

Pelargonium spp.

--

Tender annual; bring indoors

Lemon grass

Cymbopogon citrantus

--

Tender annual

Hops

Humulus Iupulus

M, P, C

.

Horehound

Marrubium vulgare

M, P, C

Lightly mulch

Hyssop

Hyssopus officinalus

C

Tender perennial

Lavender

Lavendula angustifolia

M, P, C

Hardiness varies with cultivar; mulch; provide wind protection; requires good drainage

Lemon balm

Melissa officinalis

M, P, C

Lightly mulch

Lemon verbena

Akitsua truogtkka

C

Tender perennial; bring indoors or heavily mulch

Lovage

Levisticum officinale

M, P, C

.

Marjoram

Origanum majorana

C

Tender perennial; bring indoors or heavily mulch

Mints

Mentha spp.

M, P, C

Lightly mulch; provide wind protection

Oregano

Origanum spp.

M, P, C

Hardiness varies with species

Parsley

Petroselunum crispum

---

Biennial; treat as annual

Pennyroyal

Mentha pulegium

P, C

Provide wind protection

Rosemary

Rosmarianum officinalis

P, C

Tender perennial; bring indoors or heavily mulch; requires good drainage

Sage

Salvia officinalis

M, P, C

Lightly mulch; provide wind protection

St. John's wort

Hypericum perforatum

M, P, C

.

Sweet cicely

Myrrhis odorate

M, P, C

.

Tansy

Tanacetum vulgare

M, P, C

Lightly mulch

Tarragon

Artemisia dracunculus

M, P, C

Provide wind protection; requires good drainage

Thyme

Thymus vulgaris

M, P, C

Hardiness varies with cultivar; lightly mulch; provide wind protection; requires good drainage

Valerian

Valeriana officinalis

M, P, C

Lightly mulch

Verascum

Verebascum spp.

M, P, C

.

* Normally hardy in: M = Mountains, P = Piedmont, C= Coastal Plain

Improving Winter Survival

Most herbs benefit from a 2- to 3-inch layer of organic mulch (pine straw, coco bean hulls, hardwood bark, bark and sawdust mixture) during the growing season. Mulch is an adequate winter protection for herbs such as mint, chives, and fennel providing protection to minus 20oF. A winter mulch helps maintain uniform soil temperatures around the root system and provides protection against heaving cause by frequent freezing and thawing of the soil.

Some herbs require a thicker layer of mulch to protect their roots during extended freezing weather. Heavy mulching before cold weather occurs should be avoided since it will keep the soil warmer and may actually decrease winter hardiness. After the first hard freeze, apply a 3- to 6- inch layer of organic material such as straw, pine needles, or chopped leaves. Most of the mulch should be removed in the spring as new growth begins.

Rosemary, lemon verbena, and a few other perennial herbs are not reliably winter hardy. Extra winter protection can be provided by cutting plants back to within a couple inches of the ground after the first hard frost and covering the remaining stub with soil. Then cover the soil with a 4- to 5-inch layer of mulch. For lemon verbena, the use of a microfoam ground cover (the packing material used around fragile items) also works held down with soil works very well providing over 95% survival in most years. An alternative method is to encircle the plant with a cage of hardware cloth or chicken wire. The cage diameter should be about 12 inches larger than the plant (6 inches on each side). Fill the cage with mulch.

Harsh, drying winds can prove as fatal as cold temperatures to some of the less cold tolerant herbs. Wind breaks can aid the survival and appearance of herbs such as French tarragon, germander, English lavender, Roman chamomile, and winter savory. Covering with a few evergreen boughs will prevent drying out of silver and lemon thyme foliage. The more cold-sensitive herbs have a better chance of survival if grown in a protected location.

Other cultural practices that influence winter hardiness include: fertilization, pruning, soil drainage, and watering.

Fertilizing
Herbs should not be fertilized after early August. Late summer applications of nitrogen fertilizer will promote new growth that may not have time to mature before frost. The herbs will remain actively growing instead of becoming acclimated for cold weather.

Pruning
Avoid significant pruning (light harvesting is acceptable) in August which will stimulate new growth that will not have time to mature before frost. Also, avoid severe pruning in late fall since winter hardiness is reduced until the cuts have healed. Woody plants should not be severely pruned within 4 to 6 weeks of the first severe freeze. In western North Carolina, the last severe cutting on sage, lavender, or oregano should be made before early September. Light pruning after frost is acceptable.

Soil Drainage
Excessively wet soil or sites with standing water can decrease winter hardiness of some plants. This is especially true for Mediterranean plants such as rosemary, thymes, lavenders, and French tarragon that are adapted to dry climates. Provide adequate drainage by incorporating pine bark mulch or planting in raised beds.

Watering
Keep plants adequately watered during late summer and fall. Drought stressed plants are weaker and are often less cold hardy. Water during a dry winter, especially before a severe freeze. This is especially true for evergreen plants that will lose water from their foliage on bright, sunny days even when the ground is frozen.

For Further Reading


Published by

North Carolina Cooperative Extension Service


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