How to Start Seeds Indoors in the Winter

by Erin Marissa Russell

Step one of planting a wintertime garden is starting seeds in winter, and we’re here to take you step by step through the process. Colder temperatures mean you may be starting seeds indoors when you’re used to sowing them directly in the ground. You’ll probably also need to take some extra measures to get young plants acclimated to the outdoors through cold acclimation, also called hardening off, before moving them into their permanent places in the wintertime garden.

But there’s no reason to be daunted by the extra steps it takes to start seeds indoors in the winter. Read on to find out everything you need to know to get your wintertime garden off to a strong start, with answers to your questions as well as step-by-step instructions for starting seeds indoors.

Will seeds germinate in cold weather?

Because germination (the process by which seeds sprout) takes place underneath the surface of the soil, sunlight isn’t important—but warmth is another story. Seeds germinate the best in temperatures between 60 and 75 degrees Fahrenheit. Heated mats can add 10 degrees to the air temperature if you’re raising seeds in a room that doesn’t offer enough warmth for plants to germinate.

(Note that seeds that rest on the surface of the soil may require light to germinate as well as heat. For example, begonias, impatiens, and petunias won’t sprout if they don’t get enough bright light.) When you aren’t sure of a plant’s specific needs, check the seed packet or the information provided online to determine what temperature the seeds should be kept in and how much light they need to sprout.

How do you grow seeds indoors in the winter?

Gather clean containers, such as seed trays, peat pots, or flats. As an alternative, you may choose to go DIY and add drainage holes to recyclable plastic egg cartons.

Add equal parts vermiculite and perlite to soilless peat moss to create your own soil for seed starting. This combination allows the soil to retain enough water to keep seedlings hydrated while still permitting air to circulate, and it’s fine enough to let the tender young roots of your plants spread out, unlike standard potting soil, which just won’t do for starting seeds. You can also use Jiffy pellets or another brand of pre-formed seed starters to save time and hassle if you like. Make sure that any compost you use in creating your seed starting mix has been heated to 150 degrees Fahrenheit so that weed seeds and diseases are exterminated. You can add vermicompost, but keep it to 10 percent or less of the total soil mixture by volume.

Following the instructions on your seed packet or the guidelines given on the catalog or website, sow your seeds into the prepared containers. If you’re instructed to gently press seeds into the soil, the eraser end of a pencil makes a handy tool. Selecting the largest seeds in each packet to sow will get you the highest germination success rate. Sow no more than two or three seeds per cell to prevent overcrowding.

Use plastic cling wrap to cover up your containers and keep moisture locked in, creating something similar to the greenhouse effect. Be sure to add a few holes using a toothpick or other sharp tool so air can circulate and mold doesn’t develop.

Gently and carefully water your newly sown seeds. A watering can or pitcher may release too much water for new seeds with too much force. Spraying your seeds with mist from a spray can might be gentler on them, but it takes quite a long time to get plants as much water as they need.

A good middle ground for seeds you’re starting indoors is a turkey baster or large eyedropper. Another options is a pitcher with a narrow spout, which creates a gentler stream of water. These tools will give you the water output plants need without the water pressure you’d get from a watering can or the garden hose you use on more mature plants, which can displace soil—and the seeds you’ve just planted along with it.

Keep the soil where you’re starting seeds evenly moist but not waterlogged. Check the soil where you’ve started seeds at least once per day to ensure your seedlings are getting enough water. Many gardeners will find they need to water their seedling twice per day to keep them hydrated.

When plants are just getting started, warmth is more important than sunlight, so good places to set up shop include on top of the refrigerator or close to a radiator. Plants grow best in temperatures between 60 and 75 degrees Fahrenheit.

Once you see evidence of sprouting, remove the plastic cling wrap and position plants where they’ll get 12 to 14 hours of bright sunlight per day. (Yes, that’s a lot, but young plants crave sunshine.) A south-facing window will offer more sunlight than windows that face in other direction. When plants have grown to one inch tall, thin them out by snipping the stem at the soil line to leave just one per cell for best results.

Once your seedlings have grown their second set of leaves, it’s time to transplant them. Carefully relocate young plants into individual containers filled with a potting soil that’s mixed with plenty of compost. Water transplanted seedlings thoroughly, and keep them out of  bright light for a few days while they recover from the stress of being moved.

When should I start my seeds indoors?

For most annual vegetables, as a general rule, start seeds between four weeks and eight weeks before the last frost in your area is expected. However, this timeline can fluctuate based on your location and the specific plants you’re growing. The Old Farmer’s Almanac offers a planting calendar that will provide you with specifics on when to start your seeds if you key in your ZIP code.

Which seeds should I start indoors?

In the winter, many crops benefit from getting a head start indoors, especially for gardeners in chillier climates. The plants listed below are especially common for starting indoors. When in doubt, it’s best to go ahead and start seeds indoors in winter, as a head start won’t hurt your plants—but early exposure to winter’s chilly weather can have disastrous consequences for young plants that aren’t up to the challenge.

• Beans
• Broccoli
• Brussels sprouts
• Cabbage
• Cauliflower
• Celery
• Cilantro
• Eggplant
• Kale
• Lettuce
• Moonflower
• Parsley
• Peppers
• Petunias
• Pumpkins
• Spinach
• Squash
• Swiss chard
• Tomatoes
• Watermelons

How long does it take for seeds to sprout indoors?

Germination rates will vary when you start seeds indoors based on the room temperature where your plants are kept. As a general rule, however, most plants germinate within a week or two. A few plant types, such as chili pepper, tomato, or rosemary, can take closer to three weeks to sprout.

Do you need a grow light to start seeds indoors?

A few variables come into play to determine whether or not you’ll need to use a grow light when starting seeds indoors. In short, plants require six to eight hours of sunlight per day to promote healthy growth, and they’re happiest when they get closer to 12 or 14 hours of sun each day.

If you’re starting your garden late in the winter, that much light may not be available. Or you may find that the windows in your home where your seeds get their sunlight aren’t exposed to six to eight hours of sun per day. In these cases, a grow light is needed for plants to perform their best. Where installing grow lights is not possible, you may opt instead for fluorescent shop lights housing one warm bulb and one cool bulb.

When it’s time to relocate your plants into the garden for the winter, remember not to shock them by exposing them to the cold weather suddenly. Give plants a period of transition by using the hardening off process to gradually acclimate them to the cooler temperatures. You can learn more about hardening off and get step-by-step instructions to guide you through it in this article by Gardening Channel.

Want to learn more about starting seeds indoors?

The Old Farmer’s Almanac covers When to Start Seeds Indoors

American Meadows covers How to Start Vegetable Seeds Indoors

Burpee covers Indoor Seed Starting

Country Living covers Start Seeds in Winter

The Bump covers Can Weather Conditions Affect Seed Germination

Mother Earth News covers Starting Seeds Indoors

New England Today Living covers Seed Starting in Winter

Permaculture News covers Soil Temperature and Seed Germination

Click and Grow covers How Long Does Seed Germination Take

The Spruce covers Common Mistakes Made Starting Seeds Indoors

The Spruce covers What You Need to Start Seeds Indoors