The successful outdoor growth of herbs requires precise timing. Planting herbs at the wrong time is the most frequent setback for new gardeners, often leading to stunted growth or complete loss of the plant. Success relies on aligning the herb’s specific temperature needs with your local climate conditions. Mastering this timing allows your herbs to establish a strong root system and flourish throughout the growing season.
Understanding Frost Dates
The foundational metric for spring planting is the average last spring frost date for your region. This date is determined by historical climate data and represents the average final day when air temperatures drop to \(32^\circ\text{F}\) (\(0^\circ\text{C}\)). This information is readily available through resources like NOAA or local agricultural extension offices by searching with your zip code.
Remember that this date is an average, meaning there is still an approximate 30% chance of a frost occurring after the published day. To ensure the safety of tender plants, the “safe planting date” is typically considered one to two weeks after the average last frost date. Waiting this extra period significantly reduces the risk of cold damage to young, sensitive herb seedlings.
Timing by Herb Warmth Preference
Herbs are categorized into two groups based on cold tolerance, which dictates how early they can be moved outdoors relative to the frost date. Hardy (cool-season) herbs tolerate light frost and cold soil, while tender (warm-season) herbs are highly susceptible to cold damage. Planting according to this preference ensures the plant is not exposed to damaging temperatures.
Hardy herbs, such as mint, chives, parsley, thyme, and sage, can be planted out two to four weeks before the average last frost date, or as soon as the soil is workable. These varieties thrive in cooler conditions; cilantro and dill prefer a temperature range of \(45^\circ\text{F}\) to \(75^\circ\text{F}\). Planting them early allows them to establish before the intense heat of summer, which can cause them to “bolt,” or prematurely flower and stop producing flavorful leaves.
Tender herbs, including basil, lemongrass, and summer savory, must wait until all danger of frost has completely passed. Basil is particularly sensitive, suffering damage when temperatures drop into the \(40^\circ\text{F}\) range and requiring nighttime temperatures reliably above \(50^\circ\text{F}\). The soil itself should have warmed to at least \(50^\circ\text{F}\), and ideally \(60^\circ\text{F}\) or higher, before transplanting these varieties. Rushing this planting can severely stunt growth.
The Hardening Off Process
Any herb started indoors or purchased from a greenhouse requires a gradual transition period known as hardening off before outdoor planting. This process toughens the plant’s cell structure, preparing it for the harsher elements of direct sunlight, wind, and fluctuating temperatures. Failing to harden off can result in immediate transplant shock, leading to leaf burn, wilting, or death.
The process should begin seven to fourteen days prior to the target planting date, on a day when the temperature is above \(45^\circ\text{F}\). On the first day, place the plants in a shaded, sheltered spot for only one to two hours, protecting them from direct sun and strong winds. Gradually increase their exposure time each day, adding an hour or two and moving them into slightly brighter light.
By the end of the first week, the herbs should be spending several hours a day in direct sun (if they are sun-loving varieties), but still be brought indoors at night. During the final few days, you can begin leaving them out overnight, provided nighttime temperatures remain above the minimum threshold for that specific herb. This systematic acclimatization minimizes stress and ensures the plants are robust enough to withstand the outdoor environment.
Direct Seeding Versus Transplanting
The chosen method of starting the herb influences the planting timeline, especially concerning soil temperature. Transplanting involves moving an established seedling (“start”) into the garden, while direct seeding means planting the seed directly into the final location. Soil temperature is the primary factor affecting seed germination and seedling survival.
Hardy herb seeds, like parsley and chives, can be direct-sown earlier than transplants because the seed is more tolerant of cool soil, though germination will be slow below \(50^\circ\text{F}\). For tender herbs like basil, direct seeding must wait until the soil is significantly warmer, typically in the upper \(60^\circ\text{F}\) range. This is because the seed needs greater heat for successful and rapid germination. Transplanting a start, therefore, provides a head start and is generally preferred for warm-season herbs in regions with shorter summers.