How to Plant Annual Seeds Indoors

In this article we are going to answer the question how to plant annual seeds indoors. If you don’t want to sow seeds directly into the ground, you can also sow seeds indoors. Once the seeds have sprouted into established seedlings, you can then harden off and transplant the seedlings into your garden.

Why Sow Annual Seeds Indoors?

There are two main reasons to sow seeds indoors instead of directly outdoors:

  • You control the environment: When you sow seeds outdoors, you’re at the mercy of the weather. A freak late frost, a lack of sun, or other factors could kill or damage sprouting seedlings. Indoors, you can control the temperature and the amount of water and light your plants get.
  • You can get a jump on the growing season: Because you don’t have to worry about frost, you can sow annual seeds indoors earlier in the spring than you could otherwise—up to eight weeks before the last average frost date (though exact timing depends on the specific plant, as listed on that plant’s seed packet). Then, once the last average frost day passes, you can transplant the seedlings right into your garden rather than have to start from scratch with seeds.

There are also downsides to sowing indoors. It takes time and effort to provide your sprouting seedlings with the perfect growing environment. It also takes space to store the plants and the various required supplies. Finally, some annuals don’t transplant well—you’re better off sowing those annuals directly. If an annual doesn’t transplant well or shouldn’t be sown inside, its seed packet will say so.

Supplies for Sowing Annual Seeds Indoors

To sow seeds indoors, you’ll need some tools and supplies:

  • Containers: Any 3″-deep receptacle that can hold potting soil and allow for drainage can serve as a container for your seeds. You can make your own containers by punching small drainage holes in the bottoms of egg cartons, halved milk or juice cartons, or plastic food containers, such as those used for cottage cheese or yogurt. You can also get plastic seedling containers at garden centers or online gar­dening stores. Before using any type of container for seedlings, it’s a good idea to sterilize it by washing it in the dishwasher or wiping it out with a mixture of 1 part household chlorine bleach to 10 parts water.
  • Trays: Rather than handle each container individually, place the containers on trays. The best trays have holes in the bottom that allow for drainage and also allow you to water the seedlings from the bottom when it becomes necessary to do so.
  • Fluorescent grow lamps: If you have a space in your home that can provide your seeds and seedlings with enough natural light each day, then you won’t need fluorescent lamps to give your plants extra light. But few people have ideal natural light conditions inside their homes. To sow seeds indoors, you’ll likely need to buy fluorescent grow lamps, which are sold at garden centers and online gardening retailers. You can either buy just the lights and rig them yourself over your seed trays, or you can get a light stand that holds the lights over the trays.
  • Potting soil: It’s best to grow your seeds in com­mercial potting soil rather than soil from your garden, since garden soil can contain insect pests and germs. Look for specially formulated germinating mix, which is designed to provide the best environment for sprouting seeds and seedlings. If you want to use soil from your garden, you can sterilize it by spreading it in a shallow baking pan and then heating the soil in the oven to 180°F and keeping it at that temperature for at least 30 minutes.
  • Spray bottle: A spray bottle is the best way to water seeds and seedlings. Make sure to use a spray bottle that’s dedicated exclusively to gardening and hasn’t ever been used to spray chemicals—chemical residues can harm your plants.
  • Fertilizer: Once the seeds have sprouted, you can aid their growth by applying a soluble powdered or liquid fertilizer. Look for a balanced, complete fertilizer. If you’d like to use an organic fertilizer, fish emulsion is a good choice. If possible, look for a fertilizer specially formulated for germinating plants.
  • Plastic wrap: Use this to cover the containers after the seeds have been sown, to keep in moisture.

How to Sow Annual Seeds Indoors

For most annuals, you should sow the seeds indoors anywhere from four to six weeks before the last average day of frost, though more or less time can be required depending on the species. The seed packet will provide specifics.

1.  Find a good spot: Most annuals need a stable air and soil temperature of about 70°F to sprout and grow. Make sure to locate your seed containers in an area of your home where you can control the temperature. If you want to grow seeds that require a soil tem­perature well above 70°F for germination, you may have to get a heating mat, an electric mat that you place under containers to warm the soil within them.

2. Moisten the soil: Moisten the potting soil using the spray bottle. The soil should be damp and, if molded into a ball, should hold together—but it should not be dripping wet.

3. Put the soil in the containers: Fill the containers with the soil to a height of about 1″ from the top. Keep some extra potting soil in reserve.

4. Sow the seeds: Sow the seeds in rows across the potting soil. You can sow roughly 3–4 seeds in each small container and more in each larger container. Sow only one type of seed within a container, and label that container so you know which type of seeds it contains. It’s important that you keep different types of seeds separate and label the containers so that you transplant the different seedlings at the right times.

5. Cover the seeds: Depending on the instructions on the seed packet, cover the seeds with a 1/2″ or 1/4″ layer of soil. Some seeds require light to germinate and shouldn’t be covered at all.

6. Cover the containers with plastic wrap: Put plastic wrap over the containers—this keeps in moisture and promotes sprouting. Do not keep the containers under fluorescent grow lamps at this point, unless the seed packet indicates that the seeds need light to germinate.

7. Check soil moistness: Every few days, remove the plastic wrap to check the soil’s moisture level. If the soil is dry, spray with the spray bottle until moist, then cover the container with the plastic wrap again.

8. Give the sprouts light: When you see sprouts pushing up from the soil, remove the plastic wrap. Place the container directly under the lamps, and lower the lamps so that they’re just a few inches above the seedlings. Raise the lamps as the seedlings grow, and leave the lamps on for 12–16 hours a day.

9. Water the seedlings: Once the plastic wrap is removed, the soil will lose moisture more quickly. Check the soil daily and keep it moist but not quite damp. Rather than water the seedlings using the spray bottle, at this point it’s best to water from the bottom up by dipping the tray into a sink that you’ve filled with 1–2″ of cool water. Keep the tray in the water until the moisture reaches the top of the soil.

10. Thin the seedlings: Once the seedlings are 2″ high, thin them out by pulling some of the seedlings from the soil so that the others have more room to grow.

11. Fertilize the seedlings: Fertilize the seedlings after about two weeks by dissolving the fertilizer in water and fertilizing when you water. Use the fertilizer at half the strength that the packaging recommends.

Transplant the seedlings outside based on the instructions on the seed packet and the last average day of frost in your area.