Steps for Seed Starting Success

Photo Courtesy of Robin Carlson

Here’s why we start seeds indoors

Starting our own plants allows us to control what goes on with them and is a piece of the puzzle in truly knowing where our food comes from. There are therapeutic health benefits in starting our own seedlings indoors, and it’s a task that provides a welcome boost to a gardener’s spirits during the doldrums of winter. 

Better, more diverse taste in vegetables becomes available, because starting seeds allows us to grow heirloom or unusual varieties that may not be available in the grocery stores or from plants that aren’t in nurseries. Consider the appeal of growing a “blueberry’” tomato. We can grow them ourselves by starting the seeds indoors.

The cost savings of starting our own seeds is worth mentioning. Rather than buying plants in a big box store or garden center, starting our own seeds saves money. We’re able to grow high-value crops that would otherwise be expensive to buy at the grocery store or farmers’ market. A favorite potato salad recipe might call for six different herbs and heirloom fingerling potatoes. Herbs, often sold at $7 per container, plus the cost of potatoes, has the makings of a $50 potato salad. It makes better sense to snip a few sprigs from the herbs we started indoors and then moved out into containers gathered around our back door.

Here’s the garden math

March is the time to start many seeds indoors. More precise dates can be calculated by considering exactly when the garden will be warm enough to accommodate tender seedlings. Figure out the last frost date for each gardening zone (Chicago is typically 5a or 5b); this leads to an approximation of the best planting out date and backs us into a sow date for starting seeds indoors. In the zone 5b Fruit and Vegetable Garden at Chicago Botanic Garden, we know the last frost may occur within two weeks on either side of April 25. The National Gardening Association provides a helpful link to calculate our frost-free growing season and planting calendar by zip code at

Here’s what to start

Root vegetables like carrot, beets and radish are directly sown into the garden and won’t need to be started indoors. The big seeds of peas and beans are easy to handle and grow well if sown outside at the right soil temperature, (peas need a minimum of 50-degree soils and beans need 70-degree soils). On the other hand, some vegetables don’t appreciate their roots being disturbed and transplant less well. They fall somewhere in the middle, leaving the gardener with a decision to make. While they’re best seeded directly into the garden, they could be started indoors for transplanting. Corn, cucumbers and squash fall into this category.

 Long-seasoned crops, those plants that need a long, warm growing season, are the ones to start indoors to transplant out into the garden. Tomatoes and peppers should be moved into the garden in late May or early June, when the soil is warm. Starting with strong transplants ensures plants with long maturities will have enough days in our short growing season to set fruit before fall’s first frost. Finicky germinators like parsley, broccoli and cabbage should be started indoors, where the atmosphere can be controlled.

Here’s the timing

Maturity to transplant into the garden vary by plant, and that timing affects when seedlings need to be started indoors. Below are guidelines for a few favorite vegetables, flowers and herbs.

Flowers: Pansies, poppies and snapdragons can be started indoors 12 weeks before the last average frost date. Sow calendula in flats six weeks prior and marigolds, zinnias, nasturtiums and morning glories four weeks before the last spring frost.

Herbs: Long-seasoned chives, onions and parsley need 12 to 14 weeks advance start (early March) before going into the garden. Thyme requires a head start of eight to 10 weeks. Basil, dill, cilantro and sage need a six-to-eight-week growing period.

Vegetables: Tomato, eggplant and pepper success depends on having an eight-week head start. Begin sowing broccoli, Brussels sprouts, cabbage and cauliflower indoors six weeks before the last frost date. If we forego starting cucumbers, pumpkins, squash and watermelon directly in the garden, start them indoors three weeks before their June transplant date.

Here’s what we need

Here are some tips as we gather the gear for seed starting.

Seeds: Begin with fresh seed or saved seed that’s been properly stored in a dark, temperature and humidity neutral location, such as a drawer. Seeds have expiration dates, so it’s important to read the back of the packet to check if the seed is within that date and still viable. The planting instruction on seed packets is helpful, with guides that show how deep to plant the seed (chopsticks or pencils make great tools to poke holes in the soil at the correct depth) and how long it will take to sprout. Large seeds can be planted an inch deep and small seeds need only a light covering of soil.

Soil: Buy or make a quality seed starting mix. It should be sterile, without weed seeds or pathogens that can cause problems for tender young seedlings. The mix should be finely textured, allowing the tiny new roots to push down and develop. Regular bagged potting soil becomes too packed down and shouldn’t be used. To make our own soil-less mix, use 50 percent peat and 50 percent vermiculite—no cutting corners on the soil for seedlings—soil from the garden is too heavy and may have bacteria, causing seedlings to abruptly die, a condition called damping off. Set up for success by using a sterile, soilless seedling mix such as Black Gold (

Containers: Plastic flats are easy to work with as long as there are drainage holes in the bottom. They should be at least three inches deep to encourage healthy root systems. Any container will work however; old yogurt containers and terra cotta work well, provided they have been cleaned with a solution of one part bleach to 10 parts water and rinsed well.

 After planting, the entire container can be placed in a plastic bag or the top sealed with plastic wrap to hold moisture in and aid in germination. Carryout food or salad containers with clear plastic tops make perfect mini-greenhouses for seed starting. Remember to perforate the bottom to allow free drainage so seedlings don’t drown. Remove the plastic covering as soon as the first seedlings emerge, allowing air to circulate around them.

Light: Light is very important once seedlings emerge. Late winter and early spring day length is too short to provide enough light on a window sill, so it’s a good practice to use a grow light. Fluorescent shop lights on chains can be hung two to four inches above the plants after they sprout, or full spectrum LED grow lights are also effective. Turn the lights on for 16 hours per day and off for eight to allow the seedlings to rest.

Heat: Seeds require warmth to germinate; some more than others. Peppers and tomatoes do well when flats are positioned on heat mats with a thermostatic control. This provides bottom heat to increase the temperature of the soil and results in faster germination. Try to keep the room warm, ideally 70 degrees, during the day and 60 degrees at night.

Moisture: Moisten the seed starting mix with a mister before planting the seed and maintain even moisture throughout seed germination. Streaming too hard or too much water will disturb the newly planted seed or wash it out. The light mist from a spray mister works perfectly for newly planted seeds. Maintain even moisture by checking the soil often, especially if a heat mat is used, because soil can dry out quickly. Using room temperature water is a good practice; we don’t want the seeds to struggle after a deluge of ice-cold water. Another option for keeping moisture consistent in seeding flats is by using a capillary mat. This is a great option because the water is drawn up into the tray from the bottom, so the seedlings never get wet. 

Here’s what to expect

Timing is important. Some varieties take longer to germinate, so be patient. Parsley can take up to 21 days to germinate and tomatoes just a week. If the seedlings are left too long in containers that are too small, they will become root bound. If roots are visibly growing out of the bottom of the container, this is a sign they need to be potted up into a larger container with a bit of fresh soil around their roots. 

Hardening off seedlings is a gradual transition time when trays are moved between indoors and outdoors before they can be transplanted into the garden. Over a week’s time they should be moved into a protected place out of direct sun and wind during the warmest part of the day and brought back inside for nighttime protection.

Here’s how to troubleshoot

Feed seedlings once they have a true set of leaves, usually the third or fourth leaf to emerge, using a dilute liquid feed such as fish emulsion or a low concentrations of kelp one time when watering seedlings. Yellow seedlings may be a sign of too much water or lack of nutrients. Let the flat dry out slightly, and then lightly feed them. Tall, leggy seedlings are an indication that the plants need more light and are stretching to reach it. Spindly plants can be helped by moving them into a place where they receive light for more hours during the day.

Lisa Hilgenberg is the horticulturist at the Regenstein Fruit and Vegetable Garden, in the Chicago Botanic Garden, located at 1000 Lake Cook Rd., in Glencoe. For more information, call 847-835-5440 or visit


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