The 3rd-4th week in March is a great time to start seeds in the southern half of the state. We are holding a seed starting workshop this Saturday at the Dane County Extension office. This in-person event begins with a lecture on seed starting techniques and principles followed by a hands-on session in the Dane County Extension Teaching Garden Greenhouse where attendees can practice planting seeds and will take home some seeds in packs that they have planted. There are still spaces left if you’d like to come. The class is March 16th 9am-12pm at 5201 Fen Oak Dr. in Madison. For more information and to register visit https://dane.extension.wisc.edu/events/seed-starting-workshop/ .
Starting warm-season crops like tomatoes and peppers inside like we will do in the workshop is recommended, so they bear fruit in a timely fashion since our growing season is too short for them to be direct-seeded outside. These crops should be started about 6-8 weeks before the ‘last frost date’ in your area. You can find your area’s ‘last frost date’ https://www.ncdc.noaa.gov/news/when-expect-your-last-spring-freeze . Unfortunately, that date is a median, so there is still a 50% chance of a 32°F temperature (that’s why I always put that term in quotations). I recommend putting your seedlings out a few days after that date, depending on the type of spring we are having. There are other maps NOAA offers that show a 90% probability level for the temperature dropping to 16°F, 20°F, 24°F, 28°F, 32°F, and 36°F temperatures on certain dates, and you may want to check those out as well. You can access the Freeze Normals maps by visiting the interactive map in their Climate Data Online application and selecting the Annual Climate Normals layer.
Monitor your seedlings carefully as they sprout to make sure they don’t dry out. Conversely, also make sure they are not sitting in water, as this is a great environment for damping off diseases such as pythium or phytophthora to develop. Damping off symptoms include the seedling stem withering at the base so the seedling falls over and dies due to the rotted roots below. Start fertilizing your seedlings with a dilute water-soluble fertilizer about every other week once they get to be about one inch in height. The roots should be established enough by then to be able to take up the fertilizer well. You could also use an organic fertilizer, but since they are usually less concentrated, you might need to use it more often. Check the package to see what is recommended for use on seedlings. When it gets close to the time you can plant outside, (whenever your last frost date is for your area) start to acclimate the plants by putting them outside for a while each day, starting in a semi-shaded place and gradually moving them to more sun. This process, called ‘hardening off’ should take 7-10 days at a minimum.
If you are starting seedlings under grow-lights, once they germinate, the tops of the seedlings should be kept about 1” from the fluorescent tubes. Don’t allow the seedlings to grow into the lights as the leaves may burn. Lights should be on for about 16 hours a day. Use a timer attached to your lights to set on/off times. HOWEVER, if you are growing spinach or lettuce under lights, don’t give them more than 11 hours of light since they will be stimulated to ‘bolt’ (flower). That occurs because that much light simulates the long days of summer, when flowering usually happens. When spinach and lettuce bolt, they become tough and bitter tasting. They can bolt when only 2-3” tall if they get too much light!
For seeds that get planted outdoors early, like cool season crops, don’t be too quick to put things outside. Mother Nature has a tendency to do a self-correction when temperatures are warm too early like this, and freezes or frosts are not uncommon in April and May. Also, if the soil temperature is too low, even if the air temperature is warm, the soil may be too cold for seed to germinate, even cool season crops such as spinach, Swiss chard, lettuce, radishes, peas and carrots. A seed knows when to break dormancy based on temperature. For example, yellow pear tomato ‘volunteer’ seeds dropped last fall in your garden in overripe fruit (if they survive the winter) will wait until the soil gets above 70° F before putting out a root. If you plant too early in cold soil, the seed may rot, and if it does geminate, it will be slow to do so and be likely to produce stunted plants that are more prone to disease. Give seeds the optimum soil temperature and seedlings will emerge more quickly and grow rapidly in size and strength. It’s also important not to put warm season transplants of tomatoes and peppers out too early. If the air temperature is below 55 degrees F, they will often be stunted permanently and can also develop nutritional issues since their roots can’t properly absorb nutrients from cold soil.
You can check your soil temperature by purchasing an inexpensive soil temperature thermometer. Ideally, take temperature readings on two to three consecutive mornings at the same time, usually mid-day (between 10 a.m.-12 p.m.). Insert the probe to a depth of 2 inches for seeds or 4-6 inches for transplants. v
Use the chart below to guide you on when to plant seed (measured at the two-inch depth) or set out transplants (measured at the 4-6 inch depth). Of course, there might still be a late freeze, so be ready to cover the bed if necessary.
Temperature Chart: Gives minimum temperature to plant, optimum temperature to plant, and a viable range.
| Crop Seed Type | Minimum Soil temperature | Optimum Soil temperature | Germination temperature viable range |
| beets | 45 | 85 | 40-95 |
| carrots | 45 | 80 | 40-95 |
| lettuce | 45 | 75 | 35-85 |
| parsley | 45 | 75 | 40-90 |
| radish | 45 | 85 | 40-95 |
| spinach | 45 | 70 | 35-85 |
| asparagus | 50 | 75 | 60-85 |
| peas | 50 | 75 | 40-85 |
| turnip | 50 | 85 | 40-105 |
| cabbage | 55 | 85 | 40-105 |
| cauliflower | 55 | 80 | 40-100 |
| corn | 55 | 95 | 50-105 |
| Swiss chard | 55 | 85 | 40-95 |
| onion | 60 | 75 | 35-95 |
| celery | 65 | 70 | 40-85 |
| cucumber | 65 | 95 | 50-105 |
| pepper | 65 | 85 | 60-95 |
| cantaloupe | 68 | 90 | 60-100 |
| squash | 70 | 95 | 60-100 |
| tomato | 70 | 85 | 50-95 |
| beans | 72 | 80 | 60-95 |
Have fun starting your seeds; it is a rewarding activity to nurture them and watch them grow and then reward you with food you can eat or beauty you can enjoy all summer!
