Starting herb seeds indoors is a common technique used by gardeners to gain an advantage over the growing season. This process provides a sheltered, controlled environment for seeds to germinate and develop into robust young plants before outdoor conditions are suitable. Accurate timing is important because starting too early results in oversized, weak seedlings, while starting too late negates the benefit of the head start. The goal is to produce healthy, established plants ready to thrive immediately upon being moved into the garden.
Determining Your Last Frost Date
The foundational calculation for all indoor seed starting is anchored to your average last frost date. This date marks the average point in spring when the nightly low temperature is no longer expected to dip below 32°F (0°C), the temperature considered a light frost. Since most herbs and their delicate young seedlings are susceptible to cold damage, this date serves as the primary countdown marker for planting outdoors. You can find the average last frost date for your specific location using online resources that search by zip code, such as those provided by NOAA or established gardening almanacs. Once this date is known, count backward the recommended number of weeks for each herb species to determine the ideal indoor sowing day.
Grouping Herbs by Indoor Head Start Needs
The required indoor starting time varies significantly among herbs, depending on their germination speed and growth rate. Perennial herbs and those with slow germination often require the longest head start to establish a strong root system. Rosemary is an example of a slow-growing herb, often needing to be sown 10 to 12 weeks before the last frost date because its seeds are notoriously slow to sprout.
Other slow starters, such as parsley and oregano, also benefit from an extended indoor period of eight to ten weeks before the last frost. Parsley seeds can take two to four weeks just to germinate, so soaking them overnight before planting can help speed up the process. Starting these varieties early ensures they reach a transplantable size and provide an earlier harvest.
Herbs that grow at a medium pace, like sage and chives, typically need a start of approximately eight weeks before the last frost date. These plants develop quickly but require enough time indoors to grow two to three sets of true leaves, indicating they are robust enough for transplanting. Basil, a popular warm-season annual, is highly sensitive to cold and should be started indoors about six to eight weeks before the last frost date.
Starting basil too early can lead to a leggy plant that must wait a long time indoors for the soil to warm up outside. Fast-growing herbs, such as dill, can often be direct-sown into the garden soil after the frost date has passed. If an earlier harvest is desired, dill may be started indoors with a short lead time of four to six weeks.
Maintaining Optimal Conditions Until Transplant
After sowing, seedlings must be provided with specific environmental conditions to ensure they develop into stocky, healthy transplants. The most common limiting factor is light, as natural window light is often insufficient, especially during winter months. Insufficient light causes seedlings to stretch rapidly toward the light source, resulting in weak, elongated stems, a condition known as legginess. To prevent this, seedlings require supplemental light from full-spectrum grow lights for 12 to 16 hours daily, positioned within two to six inches of the top leaves. Temperature control is also important, with most herbs preferring an ambient room temperature between 65°F and 75°F, though basil thrives closer to 75°F.
Providing bottom heat with a seedling heat mat can significantly increase germination success and speed. A soil temperature of 70°F to 75°F is ideal for most seeds to sprout. Watering should be managed carefully; the soil must be kept consistently moist but never waterlogged, as excess moisture can lead to fungal issues like damping-off disease.
The Transition to Outdoor Planting
The final step before placing herbs in the garden is “hardening off,” which acclimates tender indoor seedlings to the harsher outdoor environment. Seedlings are accustomed to consistent, mild indoor conditions and will suffer from transplant shock if moved directly outside without preparation. This shock is caused by sudden exposure to strong sunlight, wind, and fluctuating temperatures. The hardening off process should begin seven to 14 days before the scheduled transplant date.
This involves gradually introducing the seedlings to outdoor conditions, starting with one to two hours in a sheltered, shaded area when temperatures are above 45°F. Each day, the duration of outdoor exposure is increased by an hour, and the plants are slowly moved into areas with more direct sunlight and wind exposure. By the end of the transition period, the herbs should be able to remain outside overnight, provided the temperature stays above 50°F. Final transplanting should occur only after the danger of the last spring frost has passed, and for warm-weather herbs like basil, once nighttime temperatures are consistently stable and warm.