Temperature to Grow Herbs
Herbs are the name given to a wide variety of plants across many families. Usually, herbs are employed as flavoring in cooking or baking; these plants are versatile, used fresh or dried. Most herbs prefer hot temperatures to mature although some, like cilantro and mint, prefer cool and moist temperatures. All herbs require at least six to eight hours of sun. Annual herbs are classified as either warm-season or cool-season, but some herbs are perennial or biennial.
Cool-season herbs thrive in moist, cool soil and temperature conditions and can withstand normal spring frosts. Cilantro and calendula are two cool-season annual herbs that self-sow and return the next year. Sow cilantro and calendula seeds outdoors in prepared garden soil in early spring and keep them moist. Harvest cilantro before soil and air temperatures warm up higher than 65 to 70 degrees F. Cilantro will bolt and set seed once temperatures heat up. Calendula blooms well into the fall, living through several light frosts.
Annual herbs like basil or perilla require warm temperatures to germinate and to mature. Tender, warm-season herbs succumb to light frosts. Start basil seeds indoors at least six weeks prior to the last frost date in the spring. Basil seeds germinate fast at higher temperatures – within five days in soil of 70 degrees. Minimum soil temperature of 60 degrees is necessary for germination. Site basil and other heat loving annual herbs in full sun. Transplant basil seedlings outdoors when night time temperatures have warmed up.
Perennial herbs are hardy, dying back or going dormant each year in winter and returning the next spring. Examples of perennial herbs are English lavender, sage, thyme, mint, feverfew, lemon balm and winter savory. Rosemary is a perennial herb in Zones 8 and higher. Perennial seeds germinate over longer periods – approximately 21 days with temperatures from 60 to 70 degrees F. Winter hardy, perennial herbs survive late spring frosts. Most perennial herbs require full sun to mature during the growing season. Chives, hardy to USDA Plant Hardiness Zone 3, are the first herbs up in the garden in early spring and the last to succumb to hard frosts in November.
Biennial herbs like parsley grow for two years before setting seed and dying completely. In areas colder than Zone 7, overwinter parsley in pots indoors. Parsley thrives in cooler weather and like lettuce, will bolt in warm temperatures. Mullein, a biennial herb hardy to Zone 4, is a rosette of fuzzy leaves the first year. In year two, mullein sends up an 8-foot-tall flower stalk, flowers and dies.