Successful herb gardening relies on the precise timing of planting, which is determined by an herb’s tolerance for cold. Herbs are broadly categorized as tender annuals or hardy perennials, and understanding this difference dictates when they can safely be introduced to the garden. Matching the herb’s needs to the environment, whether starting seeds indoors or sowing directly, ensures a productive and flavorful harvest.
Understanding the Critical Timing Factors
The two most important indicators for successful planting are the last frost date (LFD) and the soil temperature. The LFD is an average calendar date, determined by historical weather data, that marks the last expected occurrence of a temperature drop to 32°F or below. Gardeners can find this average date for their specific location using online tools or almanacs. This date acts as a critical benchmark for heat-sensitive herbs that cannot tolerate any freezing temperatures.
Air temperature is not the only factor, as soil takes significantly longer to warm up in the spring. Soil temperature is a direct measure of the thermal energy available to stimulate seed germination and support root development. The soil temperature 2 to 4 inches below the surface lags, often remaining too cold for warm-weather seeds. Using a soil thermometer is advisable, as planting too early in cold soil can cause seeds to rot or transplants to become stunted.
When to Plant Tender and Warm-Weather Herbs
Tender herbs, which are often annuals, are highly susceptible to cold damage and must be planted only after all risk of frost has completely passed. Basil, dill, cilantro, and marjoram are classic examples of herbs that require consistently warm conditions. The safe outdoor planting window typically opens one to two weeks after the average last frost date. This delay ensures the nighttime temperatures are reliably above 50°F, a minimum these heat-loving plants require to thrive.
To ensure a productive harvest, gardeners often start these herbs indoors six to eight weeks before the anticipated outdoor planting date. This indoor start allows the seedlings to develop a robust root system and strong initial leaves in a controlled environment. Once the seedlings are ready, they cannot be moved directly outside without preparation. Sudden exposure to direct sunlight, wind, and fluctuating temperatures will cause severe stress, known as transplant shock.
Hardening Off
The transition process, called hardening off, must be performed over seven to fourteen days to gradually acclimate the young plants. Begin by placing the seedlings outdoors in a sheltered, shaded location for one to two hours on the first day. Each subsequent day, increase the amount of time they spend outside while progressively exposing them to more direct sunlight and wind. This slow conditioning toughens the plant and prepares the herb for its permanent home.
When to Plant Hardy and Cool-Weather Herbs
Hardy herbs, including many perennials, are much more forgiving of cold temperatures and can be planted earlier in the season. Mint, chives, thyme, oregano, and parsley can often be planted as soon as the soil becomes workable in early spring. These herbs can tolerate a light frost and prefer the cooler shoulder season before the intense heat of summer arrives.
Starting these herbs from seed is possible, though their germination times vary significantly. Parsley is notoriously slow to sprout, but soaking the seeds overnight can help speed the process. Thyme and oregano seeds require light for germination, meaning they should be sown on the soil surface and only lightly pressed in. Many gardeners opt to purchase small starter plants or divide existing perennial clumps to accelerate their garden establishment.
Some hardy herbs, such as chives, can be easily direct-sown into the garden once the soil thaws. Others like thyme and oregano benefit from an indoor start eight to ten weeks before the last frost date. Many hardy herbs also feature a secondary planting window in early fall, which allows the plants to establish their root systems before the ground freezes, leading to vigorous growth the following spring.