By Jessica Carlson

OTC Master Gardener

Starting seeds indoors is one of the most practical ways Minnesota gardeners can make the most of a short growing season. In USDA Zone 4a, we typically have about 110 to 120 frost-free days, often stretching from mid-May to mid-September. That limited window makes planning ahead essential, especially for warm season vegetables and annual flowers that need time to mature before fall frost returns.

Crops such as tomatoes, peppers, eggplant, and many annual flowers benefit greatly from an indoor start because they require warm soil and longer growing periods. Some cool-season vegetables, including broccoli, cabbage, and head lettuce, can also be started indoors to produce sturdy transplants ready to go into the garden as soon as conditions allow. When choosing seeds, carefully review the information provided on the seed packet or, if purchasing online, thoroughly read the descriptions of the selected variety. Look for days to germinate and days to harvest and consider whether the variety is recommended for short growing seasons. In Zone 4a, selecting quick maturing plant varieties can help ensure a successful harvest before frost arrives. The University of Minnesota’s seed trials offer comprehensive information on reliable, short-season vegetable varieties appropriate for Minnesota gardens; details are accessible through their website.

For indoor seed starting, choose a spot with consistent light. A sunny windowsill usually isn’t enough, causing weak growth. Use artificial lights, such as fluorescent or LED grow lights, to help seedlings thrive by placing them about 2 inches above the tops of your seedlings. Plants need 12 to 16 hours of light daily. Don’t leave the lights on continuously, as many plants need some dark to develop properly. A simple timer can be part of the set up so the lights can turn off and on automatically. Bottom heat from a heat mat aids germination, especially in cooler homes. Choose a sterile, soilless mix for drainage and disease prevention, and ensure containers have drainage holes which help keep moisture even, not soggy. 

Cold sowing, or winter sowing, is an alternative for some plants. This method uses outdoor containers to expose seeds naturally to winter temperatures, allowing cold tolerant plants to germinate when conditions are right. Cold sowing is effective for hardy greens, peas, and native or perennial flowers suited to cool spring conditions. The University of Minnesota Extension notes that this method reduces the need for indoor space and produces seedlings already acclimated to outdoor conditions.

Hardening off is the final step before outdoor planting. Gradually expose seedlings to outdoor conditions over 7–10 days. Check both air and soil temperatures with a thermometer placed 2–4 inches deep before transplanting. Warm season crops like tomatoes and peppers should not be planted until soil temperatures reach at least 60 to 65 degrees. Planting into cold soil can stunt roots, slow growth, and undo weeks of careful indoor work. Minnesota gardeners can monitor soil conditions using the same tool many farmers rely on – the Minnesota Department of Agriculture’s Six-Inch Soil Temperature Map. This resource shows real time soil temperatures across the state and helps gardeners decide when it’s truly safe to plant. The map can be found at https://www.mda.state.mn.us/protecting/soilprotection/soiltemp.

With careful seed selection, proper indoor conditions, and attention to soil temperature, starting seeds indoors helps Minnesota gardeners begin the season successfully. For more information on seed starting indoors visit https://extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors