Peppers, eggplants, melons…the world is full of delicious fruits and vegetables that can’t grow to maturity in a short temperate growing season. This means that unless they want to spend extra for nursery-grown seedlings, gardeners in colder climates have to start these plants from seed indoors and transplant them after the soil warms up. 

Starting your own seedlings can seem daunting if you have never tried it. Fortunately, as long as you place seeds in soil or a growing medium, then give them warmth and moisture and light, you are bound to have some success. Of course, a little extra knowledge will give you the best chance at healthy, robust seedlings that will give you a harvest before the first frost of autumn sets in.

When are first and last frost?

Seed companies and almanacs publish the average dates on which each region can expect the last frost of winter/spring and the first frost of autumn/winter to occur. The frost-free time between these dates is your growing season. While plenty of seeds can be sown directly in the garden before the last frost, it is not safe to transplant most seedlings outside when there is still a chance that the temperature will drop below freezing. When this happens, the water in plants’ cells will turn into ice crystals that can puncture their own cell walls, turning their beautiful, green leaves into soggy, blackened mush. 

It’s useful to count the number of days in your growing season so that you will know which seeds require more growing time than your climate naturally offers.

When should you plant your seeds?

Consulting experienced gardeners in your region (or Google) is an easy shortcut, but you can also do this calculation yourself. Your seed packet will usually tell you how many days your seeds will take to germinate, and how many days they will need between germination and harvest. Add those two numbers together. If your total is 75, then you will know that you need to start your seeds at least 75 days before your region’s first frost date.

How to prepare your seeds.

Plenty of seeds can go straight from packet to soil with no fuss, but you can sometimes improve your rate of germination (the number of seeds that actually sprout) by mimicking the natural conditions that would trigger growth in the seed’s natural habitat. All seeds have evolved mechanisms to delay germination until conditions are right—otherwise they would all sprout immediately around their parent plant, and in the worst possible season. The techniques below help us speak the seeds’ language and tell them it’s springtime.

Scarification and Soaking

Is your seed big, and hard, perhaps wrinkly? It has evolved to grow only when prolonged soaking lets water penetrate its thick seed coat. To help it along, gently nick the seed coat with a knife, nail file, or sandpaper, then place the seeds in a shallow bowl and cover them with warm water. Plan to plant them in soil eight to twelve hours later. 

Seeds suited to scarification and soaking: peas, beans, corn, pumpkins, squash, sunflowers, lupines, sweet peas, and nasturtiums

Stratification

Is your seed from a plant native to a climate with cold winters? Then it expects to undergo a period of cold before it’s safe to sprout. Make sure your seeds are completely dry, then place them in an airtight container in the back of the refrigerator, where they will get the most consistent temperature. After a few weeks, take your seeds out and let them sit at room temperature for twenty-four hours before planting them.

Seeds suited to stratification: wildflowers, perennial flowers, and herbs native to cold climates

Pre-Sprouting

Are you unsure if your seeds are still viable (able to sprout)? Seeds can endure thousands of years in the right conditions, but if you aren’t certain yours were stored properly it can be disheartening to watch a pot of soil and wonder whether anything is happening below the surface. Fortunately, it’s extremely easy to test your seeds and give them a jump-start on germination. This technique is usually done with paper towels and plastic sandwich bags, but for a smaller impact on the waste stream, newspaper and a clean, repurposed plastic bag works just as well.

In a single layer and with spaces between them, lay your seeds between a few pieces of dampened newspaper. Place the newspaper in a plastic bag. Leave the bag open for air circulation and place it in a warm place for a few days. Check it daily, keeping the moisture consistent but not so damp that fungus takes hold. When you see roots emerge, gently lay each seed on its side on top of thoroughly dampened soil, then place a little more soil on top. The root will dig downward of its own accord, seeking moisture. 

What kind of soil is best for sprouting seeds?

The truth is that seeds can sprout in any soil, including a bucketful scooped from your own garden in the autumn and stored in a dry place. However, potting mixes and soilless growth mediums are popular because they are sterile (there are no other seeds, moulds, or fungi present in them), and it’s easy to keep them at a steady moisture level. The most common materials that give potting mixes this second quality are sphagnum moss (also known as peat moss), which is not a sustainable resource, and coir, which is fibre from coconut husks that are transported to temperate climates using fossil fuels. Gardeners must make their own choices about what option is best for them.

If you want to avoid peat moss and coir but are reluctant to deal with seeds and spores that may be lurking in your garden soil, you can purchase sterile topsoil from a garden centre or sterilize your own in a regular oven. 

Prepare your pots for sprouting seeds.

Start by pouring your soil into a large container and working out any clumps. Add water until it is just damp enough to form clumps when you squeeze a handful.

Any pot or tray will do for starting seedlings, as long as it has drainage holes to let excess water drip out. Egg cartons and pots made from newspaper or other porous material also work, because the water can travel straight through. Loosely fill your pot with your pre-dampened soil, then hold it a few finger widths above your work surface and drop it. Do this several times. This will collapse any air pockets without compacting the soil too much. When you’re done, your pot should be about four-fifths full. 

How many seeds to plant in each pot or cell.

It is a good practice to plant a few seeds in each pot or cell. Usually, space is at more of a premium than seeds are, and you don’t want to have a wasted pot with nothing growing in it. However, be prepared to thin your close-growing seedlings by pinching off the smallest or weakest of the group. If you are worried that you won’t be able to harden your heart enough to perform this essential chore, it’s better to just plant one seed rather than have your plants fail later because they are growing too close together.

How to take care of your newly planted seeds.

At this stage, your seeds don’t need light. They do need consistent dampness and warmth. The top of an appliance like a refrigerator is often a good place. A spot near a vent is usually not, since the moving air will dry out the soil. Spray the soil regularly with water to keep it damp, or pour water in the tray beneath your pots so that the soil can soak up the water from below. You can also lay plastic over the pots to prevent evaporation, but lift it at least once a day to allow some air exchange and prevent mould. 

Once your seeds begin to germinate, you will see their cotyledons, or seed leaves, push through the surface of the soil. These leaves were contained within the seed and will begin the process of photosynthesis until the plant can grow its first true leaves. Most cotyledons are oblong or heart-shaped and will fall off later when the plant has enough true leaves to keep it going. 

Now your plants need light. Your sunniest window will do, but for the most vigorous growth possible, your seedlings should have fourteen to sixteen hours of light each day. Since that isn’t possible in a temperate springtime, and since few windows get direct sun all day, you can supplement with fluorescent light. You can find specialty full-spectrum lights made for seedlings, but it’s also effective to buy one cool white fluorescent light and one warm yellow one. Most hardware stores will have both kinds. 

This is the single most important tip contained within this post: Hang your lights so that they are always about ten centimeters (four inches) above the tops of your seedlings. This will prevent the seedlings from stretching to find sunlight, and will, therefore, make them sturdier and more resilient for life. If you can’t do this, and if your seedlings do stretch and bend to make the most of their light source, rotate them often so that they don’t stay bent in one direction all the time.

When you water your seedlings, do it gently so that the roots are not disturbed. Spray bottles, turkey basters, and spoons are all viable tools, and bottom-watering from the tray beneath your pots is also excellent. Whichever method you use, make sure the soil is damp all the way to the bottom. If water is usually found only near the top of the soil, the plant’s root will grow upward and it won’t be a solid anchor. Water regularly, but now that your seeds have sprouted you should let the soil get nearly dry between waterings. This prevents dampening off, a condition in which a fungus grows in the wet soil and destroys the plant’s root. It’s particularly important to be vigilant on this front when you are using regular soil, which is denser than potting mix and has less airflow. 

To prepare your seedlings for their future lives outdoors, blow on them often or run a fan so that the moving air just barely makes them sway. This will ensure that their first encounter with wind is less of a shock.

How to thin your seedlings.

If you have multiple seedlings growing close together, you need to thin some of them out to give the survivors room to grow. It can be tempting to try and separate them so that both plants can live, but disturbing their roots in this way can result in both plants dying. The best thing to do is to use scissors or pinch with your fingernails to cut the smaller, weaker plants off low on their stem. Do not pull them out of the soil, as this will disturb the roots of the seedling you want to keep. 

Re-potting your seedlings.

If you have used small pots or cells, you may need to repot your seedlings before it’s time to put them in the garden. This is the case if several weeks have passed since germination; if their leaves are crowding each other; and especially if the roots are running out of space. You can check the roots by placing your fingers on the soil at either side of a plant’s stem and tilting the pot until the whole root ball comes out. If you see roots wrapping around the soil, it’s time for a bigger pot.

Partially fill your new pot with pre-dampened soil and place your seedling’s root ball in the centre. Hold it steady while you fill in the space around it with more damp soil. For most plants, the top of the root ball should be even with the top of the new soil. However, tomatoes should be buried up to their lowest set of leaves. Tomatoes have the ability to grow new roots from their stems when buried, and they benefit greatly from the practice since it makes them less spindly over time. 

Gently press in your new soil, but don’t compact it. 

Condition and transplant your seedlings.

After weeks of careful tending, it can be heartbreaking to plant out your seedlings only to see them waste away. These plants, coddled their whole lives indoors, are not used to the elements and can die of shock or sunburn if their environment changes too suddenly. To prevent this, they need to undergo a process called hardening off. 

Start by taking your seedlings outside for part of the day. Place them in a sheltered spot where they will not experience direct sunlight, wind, or rainfall. Floating row cover is a product that makes an excellent shelter for hardening off plants. At night, or if there is bad weather, bring them back inside. You can also gradually increase the amount of time they spend outside.

After about a week of this treatment, your seedlings are ready to plant. The ideal conditions are a dry, cloudy morning, or a dry, sunny afternoon. That way they won’t get too much direct sun in their first day. Water the seedlings, then wait and water them again. Transplanting is traumatic for plants, and being thoroughly damp helps. 

Dig your holes deeper than you need, and return some soil to the bottom. This ensures that the area around your plant’s root system is not compacted. Fill the hole with water and let much of it drain away, so that the roots will spread downward in search of the water. As with repotting, make sure the top of the root ball is even with the top of the hole, except in the case of tomatoes. Press down lightly to remove any air pockets, but not so much that you compact the soil. 

Taking care of newly transplanted seedlings.

Make sure to water your transplants regularly, especially in the beginning as they are getting established. Pay attention to weather reports in case an unexpected late frost is announced. If that happens, protect your plants by simply covering them with upturned pots or floating row cover overnight. 

Congratulations: you now know enough to defy your short growing season and start a successful crop of seedlings indoors. What are you waiting for? Grow on!