Starting seeds indoors is an effective way to maximize the short, frost-free period available in Missouri gardens, allowing gardeners to harvest earlier and grow varieties that require a longer season. This practice gives slow-growing plants a substantial head start in a controlled environment before the unpredictable spring weather arrives. Successful indoor seed starting depends on precise, location-specific timing to ensure seedlings are mature when outdoor conditions become suitable for transplanting. Determining the correct indoor sowing date requires understanding the state’s varied climate and accurately calculating the final spring frost date for a specific location.
Defining Missouri’s Planting Window
Missouri’s large geographic area encompasses a wide range of climates, which translates into significant differences in the length of the growing season. The state primarily spans USDA Hardiness Zones 5b, 6a, 6b, and 7a, with a small portion of the southeastern Bootheel reaching the warmer 7b and 8a zones. These zones reflect the average extreme minimum winter temperature.
The growing season is much shorter in northern Zone 5b than in southern Zone 7a, creating different timelines for planting. The varying geography, including the higher elevations of the Ozark Plateau, makes the risk of a late spring frost highly localized. Gardeners must identify their specific zone to select the appropriate plants and calculate the most accurate date for outdoor planting.
The Last Frost Date Calculation Method
The most important piece of data for timing indoor seed starting is the average last spring frost date (LFD) for a specific area. This date represents the point after which there is a low statistical probability of a temperature drop below 32°F. In Missouri, this average date can span nearly a month, ranging from early April in the southeastern Bootheel to late April in the Ozarks or Northern Missouri.
To determine the correct indoor start date, gardeners must first find the local average LFD, often available through local extension offices or online calculators. Once this date is established, the gardener counts backward the number of weeks recommended on the seed packet for a particular crop. For example, a seed requiring eight weeks of indoor growth must be sown eight weeks before the local average LFD.
Timing Guidelines for Common Crops
Indoor seed starting is generally reserved for crops that require a long growing season or are highly sensitive to spring cold, particularly the heat-loving nightshade family.
Warm-Season Crops
The earliest seeds sown are long-season, warm-weather crops like peppers and eggplant, which require a substantial head start. These are typically sown indoors 8 to 10 weeks before the average last frost date (LFD) to ensure they are mature enough to produce fruit quickly once transplanted. Tomatoes grow slightly faster than peppers and eggplant and are best started indoors 6 to 8 weeks before the LFD. Starting them too early can lead to leggy, root-bound plants that suffer greater transplant shock.
Cool-Season Crops
Cool-season crops, such as broccoli and cabbage (Brassicas), need a moderate head start. They are typically started indoors 4 to 6 weeks before the last frost. These seedlings tolerate cooler spring temperatures and can often be transplanted outdoors a few weeks before the LFD.
Crops Not to Start Indoors
Certain vegetables should not be started indoors due to delicate root systems or rapid growth rates. Plants like corn, beans, and the squash family (including cucumbers and zucchini) are best direct-sown into the garden soil after the danger of frost has passed. These crops are highly susceptible to transplant shock and often struggle to establish themselves after being moved from an indoor container.
Preparing Seedlings for the Outdoors
Once the calculated indoor growing period is complete, and the local average last frost date is near, seedlings must undergo a crucial transition process known as “hardening off.” This step prepares the young plants for the harsh, fluctuating conditions of the outdoor environment, which include intense sunlight, wind, and temperature shifts. Without proper acclimation, tender seedlings will suffer severe sunburn, wind damage, and stunted growth, often leading to transplant failure.
Hardening off should begin about 7 to 14 days before the planned final transplant date. This involves gradually exposing the seedlings to outdoor conditions, starting with a few hours in a shaded, sheltered location. Over the next week or two, the time spent outdoors is slowly increased, exposing the plants to progressively more direct sunlight and wind. The final transplant date should occur only after the risk of frost has passed and the soil temperature is warm enough.