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A Seed Starting Primer

seeds germinating under plastic dome

Start your own plants … soon

Why start seeds indoors?

There are certainly plenty of vegetables and flowers you can direct-sow in your garden. However, for those of us who live in the Northern US and like warm-weather crops, many of our favorite plants to grow— and eat —require too many weeks of both warmth and long daylight to wait to sow until after frost. And for those of you who have been buying plants, starting your own seeds can both save you money and allow you to control which varieties you plant.


Supplementing daylight with artificial light, along with controlling temperature, is nearly always necessary to start seeds at home. A window with an ideal Southern exposure is unlikely to provide enough light, although some might provide close to enough, especially as we move toward longer days.

A basic setup usually includes at least one “shop light” or florescent work light from the hardware store (these start around $12 for a 2-light model; select a model using the new standard, more efficient T-8 bulbs, if possible) and a couple of full-spectrum tubes. (Full-spectrum tubes can be expensive or hard to find. I use 5000K “daylight” tubes and they work fine; some people recommend using one cool-white and one warm-white bulb.) You will need a way to control the distance of the light from the tops of the plants, ideally keeping the tubes just 1” to 3” away from the top of the growing plants and moving them up as they grow. I add white reflective materials around my plants to reflect light in and around the seedlings as well. In a basement you may be able to attach the light’s chains to pipes in your ceiling, or you may be able to install hooks; there are simple directions for frames you can make from PVC pipe and fittings online here and here as well.

 Light Stand

Many seed varieties appreciate bottom heat to start, but not too much heat. While I've successfully started seeds on a platform raised above a radiator, you will get much more consistent results with a propagation heating mat. These start around $24, and go up–way up! That said, I have a couple I have used for about 25 years, so think of them as an investment. DIY types have used incandescent bulbs or strings of old holiday lights, but with water ever present, I'd buy the mats.

Plants do appreciate cool and dark nights of about 8 to 10 hours, so you'll also need a timer to turn all this equipment off unless you are vigilant and keep perfect hours. A power strip plugged into air conditioner timer is ideal to handle the load and accommodate the grounding 3-spade plug found on some equipment.

As the variety of plants you grow from seed increases, you will find you need strip lights at different heights and areas with and without bottom heat. My setup uses two 3-lamp 8' shop lights hung in parallel and two separate heating mats covering 60% of the lit area. This gives me the variety of conditions to get me through most of the seed-starting season. I supplement this with some window locations as well!

Dates and timing

Speaking of varieties of plants, each type of plant will have an ideal sowing date based on the number of weeks before our frost date. (90% of the time, May 15 is Evanston's last day of frost. Our median date for last 32-degree frost is 4/26, but the latest date is 5/26, so most people use the 90% date. You may have your own date you use for planting-out frost-tender plants.) If you were to choose one sowing date, it would be eight-weeks before last frost, because that's what tomatoes and peppers require! But eggplants require more time, about 10 weeks, and okra requires just 6 weeks so the plants don't get too large. I even like to start some direct-sow-able vegetables inside just a few weeks before planting dates, such as bush beans and winter squash, because the germination rates is much better when you can avoid cold, wet soil. Refer to your seed packets, catalogs, or your favorite book for advice, but remember to use our local frost-free date when creating your start-date calendar!

Once I've decided what I want to grow, I go thorough my seeds and check for viability (see Winter Seed Selection article to learn how to test or find out how long specific seeds generally remain viable on the chart (Acrobat PDF) on our website) and order what I need in time for sowing dates. Once I have everything in hand, I either physically sort the packet by the sowing dates or make a chart or mark a calendar with what I will need to sow when. If you have a germination problem or a later problem like damp-off disease, there may be time to start another set because plants started a little late will often catch up due to longer day lengths available.


You'll need soil, trays for your pots, pots, and a way to control loss of moisture for the germination period. The most essential ingredient for successful seed starting is sterile seed starting planting medium. Starting seeds that later suffer from damp-off disease is frustrating, and sterile medium is a worthwhile investment. It's possible to make your own. Some people also swear by worm compost or other well-rotted compost.

You can look at pots and trays as recyclable supplies or as equipment, since you can get may years out of most. I keep every “six” pack and other plastic pot from any plants I do end up buying, so I have quite a collection. I recycle the worst of them every year, but most have served me many years. I've also added plenty of containers not designs as pots to my collection. Some people like yogurt cups. My favorite is tofu containers, which I find easy pierce with a knife to add drainage slits. (without holes they also make good trays for a single six-pack.) They also are sized well to hold toilet-paper tubes cut in half. These cardboard tubes work like expensive peat “Jiffy” pots–you can plant the entire tube to avoid disturbing roots of sensitive plants and they will degrade in the soil. Other options include home-made newspaper pots and soil blocks—which eliminate the pot entirely.

In all cases, these should have plenty of drainage and fit well in a tray. You will be watering your seedlings from the bottom. I find it worth investing in special-purpose flat trays with dome lids like these from Johnny's Seeds. It's also possible to get kits. I have more trays than lids, because you just need to keep the lid on for extra high humidity only until true leaves have forms. (The first leaves a seed sprouts are called seed leaves and seldom look anything like the plant's true leaves.) I used to use plastic bags, but the domes can be washed in bleach solution to avoid disease and don't collapse and sit on the soil or the plant leaves as bags frequently do, which may lead to disease and rot problems. That said, clean bags, especially with a large plant tag to hold the bag upright, do work.

Labels and tags: It's critical to label your pots well. Include the type, variety and sowing date. You may even wish to include any seed depth or transplant data ahead of time so you don't have to refer to the packet later. Plastic plant tags work best, but craft sticks or even adhesive labels can work. Pencil is the best way to mark tags because it does not run or fade as easily as anything else.

Setup: Perhaps the least pleasant tasks is being sure everything is extremely clean to avoid disease. Wash everything well in dilute (1/10) bleach solution and then rinse well. For those not procrastinators, doing this job in the summer and storing everything after it's all clean is the best way to go!


Seed staring mix tends to have a lot of peat moss, which requires warm water to get damp (when dried out it's hydrophobic, but will absorb warm water). Therefore it's important to dampen and mix all your soil-less mix before filling pots. When you do fill them, fill them completely to the top and then very lightly tamp them down or settle them by banging. You do not want air pockets, but you want an open structure for easy root penetration. Either under fill containers or use your finger, a dibble, or a pencil to create pockets at the right depth for the seed type. In any case, you will top-them off with fine starting mix to cover the seeds. Open packets cleanly with scissors (unless they have a reclosable flap), create a crease down the center of the packet, and tap the seed packet to form a single-file line of seeds down the crease. By lightly tapping the back of the packet you should be able to deliver a single seed (most of the time!) where you want it. It takes practice, so start with easy seeds—those bigger and not perfectly round! The number of seeds you put in each cell depends on the predicted germination rate, how much space you have, and your tolerance for pinching out the “weakest” one later. One to three per cell is a normal range.

Label the pot, place it in a tray with water, and cover or bag. (No fertilizer is needed at least until transplant time.)

Beyond the number of weeks before your transplant date, it's important to learn about each species’ idiosyncrasies. Some seeds, including many lettuce seeds and quite a few popular flowers, must be sown on or just below the surface because light is a requirement for germination (below is suggested because otherwise the seeds dry out and light does penetrate a thin layer of soil). Seeds require different temperature ranges for germination, with a few requiring cool temperatures. And lettuce seeds, again, need to be pre-chilled in a process called Stratification. Stratification (which can be done in your freezer or refrigerator) should not be confused with Scarification, which means chipping into or sanding away a bit of the hard seed coat. Okra benefits from this, and parsley does, too; however the common way of dealing with parsley is to soak it in warm water for 24 hours before sowing.

A common thing to consider is knowing which plants really hate having their roots disturbed. Chief among these is anything in the squash family—cucumbers, melons, pumpkins, winter and summer squash, which is one good reason to direct sow squash. (The other is it will not make any progress unless the soil is truly warm.) For these seeds, use the toilet paper tubes or peat pots or anything where you can plant the pot.


With your heat and light requirements controlled by timer, the grower is mostly concerned with keeping the soil moist by adding water to the tray, and raising lights when the plants grow. Seedlings also require decent air circulation to keep healthy, so you may need to rearrange them from time to time, and certainly uncover them gradually once they have true leaves. Keeping a daily watch is necessary, and is also a lot of fun.

If multiple seeds germinated in a cell, you will have to pinch out the weakest or transplant them to separate cells before too long. In any case, once the cells get root-bound, you will need to pot individual seedlings up to a larger pot. Some species can avoid this, but stronger plants often result from transplanted seedlings. They are also much less likely to dry out and will transplant to the garden with less shock. I admit I often don't re-pot as many seedlings as I should, because they require more space and more soil, more labels and more work! However, I know the results are also better. Therefore, the lesson I should learn is to start fewer plants and transplant a larger percentage. Maybe this year!

Almost into the garden

Before planting time, you will need to harden your plants off. This accustoms them not only to the range of temperatures in the real world, but scorching sun and drying winds. Therefore you will need to find an intermediate place and short periods of extremes. A shady spot with some wind protection on cool days works well. Growth slows slightly and foliage will thicken when the plants harden off.

Then pull out your garden plan and begin transplanting your home-grown seedlings! Good luck.