How and When to Start Seeds Indoors
Growing seeds indoors is perfect for sensitive and picky plants. Many seeds benefit from a sheltered environment in your home before they succeed outdoors. Learn when to start seeds inside, how to grow them, and what to do with the seedlings in this guide that takes you through each step of the process.
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Seed starting is an exciting experiment! Each seed you place into a pot is a placeholder for mature vegetables and flowers next year. While all else is quiet, cold, and dark outdoors for the winter, your seed starting hub will be alive with activity.
Indoor seed starting is perfect for gardeners looking to get a head start on their growing season. You’ll have mature seedlings ready for transplanting while outdoor sowings are still germinating. Other plants prefer growing outdoors, like those that hate root disturbance or need cold temperatures to thrive.
A greenhouse, cold frame, or row cover is similar to indoor seed sowing, as you’ll have more control over the germination process than with the outdoors. Use these steps below to plant seeds no matter what structure you sow them in. A greenhouse, laundry room, or bathroom will do—use whatever you can!
If you’re unsure when to start your seedlings, consult the indoor sowing guide, below. Ideal times are listed in relation to your region’s last frost date. To determine your last frost date, input your zip code into the National Gardening Association’s frost date calculator.
When to Start Seeds Indoors
Before starting seeds it’s important to know when is best to sow them. The optimal sowing time depends on your climate, how much frost your garden receives, and what season you’re growing plants in. Once you know the optimal sowing time you can begin sprouting seedlings to no end!
Start in Fall or Spring?
Fall and spring present ideal conditions for growing cool-loving crops. Growers with mild winters often overwinter veggies like cabbage, kale, and winter peas. Others with harsh winters start most crops indoors for the spring and summer growing seasons.
You’ll want to start fall crops indoors starting in midsummer. They’ll grow their first few leaves inside your home. Then, when they’re ready for transplanting, you’ll transition them into cool weather outside. This is best for vegetables like kohlrabi, celery, and cauliflower.
Spring is ideal for planting most other crops. Start the plants indoors before your first frost date according to how many days they need to reach a transplantable size. Even in warm USDA hardiness zones 9 and above, indoor seed starting gives you a head start on the growing seasons.
Consider Days to Maturity
“Days to maturity” is a rating on seed packets that tells you how long your flowers, herbs, or vegetables need to reach a harvestable size. Crops like broccoli and sorrel reach this size quickly, while plants like peppers and eggplants need months of growth before they produce fruits.
If your peppers need 85 days to reach maturity, plant them indoors eight to ten weeks before your last frost date. This gives them ample time to grow while allowing you two weeks to transition them outdoors.
Transplant most seedlings after the danger of frost passes, or two to four months before the first frost date in the fall. Cool weather-loving species like watercress, Swiss chard, and Brussels sprouts may go outside a week or two before the final frost, as they’re tolerant of short cold spells.
You don’t have to match the days perfectly, but it helps to consider how long your seedlings need indoors before you transition them outdoors.
Watch for Average Frost Dates
The easiest way to know when to start seeds indoors is to consider the final and first average frost dates of your region. Once you’ve determined your dates (see the frost date calculator linked above), coordinate those days with the indoor seed sowing guide we provide in this article.
Your last frost date is the first day of spring when there is less than a 50% chance a frost at 32 degrees or less will occur. It’s a good indicator that it’s time to transplant plants that prefer cool weather, while it’s best to wait a week or two before moving warm-loving crops.
You’ll start thinking about the last frost day in early January, February, or March. While snow falls outside you can sow peppers, eggplants, and bulbing onions. Two months before the final frost day you’ll sow herbs, tomatoes, and collards. In the final month before the date, start hot weather lovers like melons, pumpkins, cucumbers, and squash.
Then, as summer breezes by, you’ll begin to consider your first fall frost day for autumn harvests. This signals the beginning of winter and spells the end of the growing season for most gardeners. Warm climate growers are lucky—you can sow cool climate-loving seeds throughout the cold months.
How to Start Seeds Indoors
Now that you know when to start seeds, let’s learn how to sow them! We’ll go step by step through the process so you know exactly what you need to grow the vegetables, herbs, and flowers of your choice.
Step 1: Gather Materials
Starting seeds indoors requires more materials than outdoor sowings do. It’s simply because you’re creating the natural conditions for your seeds to grow, rather than relying on nature to do the work for you. You’ll influence the indoor climate, water levels, and the type of soil the seedlings grow in.
These are the essential tools and materials you need to grow seedlings indoors:
- Pots
- Trays
- Seed Starting Soil Mix
- Seeds
- Watering Can or Spray Bottle
While not necessary, these additional tools help boost your seeds’ germination rates:
- Heat Mat
- Grow Lights
- Fan
Plants like peppers, eggplants, and tomatoes need warm temperatures to germinate. A heat mat ensures they stay as warm as they need throughout the process. Grow lights are also not necessary, but they’re incredibly helpful in dark houses or regions with short winter days. You can grow seedlings anywhere with a set of grow lights!
The fan is perfect for creating airflow in musty rooms or areas with stagnant air. It’s not necessary if there’s already ample airflow, or if your growing hub is near a window. Open the window daily and you’re set! Otherwise, invest in a small fan to place near the potted plants. Have the air blow near the seedlings and not directly at them.
Step 2: Moisten the Soil
Premoistening the soil makes seed planting a simple task. Dry soil may shift around when you water it, causing little seeds to float to the surface. Moisten it ahead of time so the dirt’s ready for planting and easy to handle.
Mix some water in the bag or bucket of soil until it’s moist but not soggy. Absorbent materials like peat moss, compost, and coco coir require more moisture than less absorbent things like perlite or vermiculite. Adjust the amount of water to match your soil mix.
The best seed-starting mixes have adequate drainage and are absorbent and porous. Use a mix made for sowing seeds or create a blend. If you’re making a soil mix, try this ratio as recommended by Steven Cornett from Nature’s Always Right:
- 1/3 Compost or leaf mold
- 1/3 Perlite or vermiculite
- 1/3 Coco coir, peat moss, or similar material
Peat moss isn’t a sustainable material and its harvesting contributes to climate change, so consider trying alternatives like coco coir, rice hulls, straw, or decomposed wood products. Chopped leaves also work well if you cut them up into tiny pieces. Use a lawn mower or weed wacker to make quick work of a pile of whole leaves.
Step 3: Prepare Pots in a Good Location
Drafts may present issues for your seedlings if they’re too cold or hot. Hot air is dry and sucks moisture out of the room, while cool air chills sensitive plants. Grant the seedlings a protected cove near a bright window or grow lights. If the room is particularly chilly, consider adding a heat mat and germination domes to keep the seedlings moist, warm, and cozy.
Prepare your cove by filling the pots with seed-starting soil. Leave some room at the top so water doesn’t overflow when you water your plants. Tap the pots down on the floor or in their tray to help the soil settle. If it settles far below the rim, keep adding the moist mix until it reaches near the top.
Step 4: Sow Seeds
Before sowing your plants of choice, first, see if they need any pretreatment. Some large seeds like nasturtiums or peas benefit from scarification. This involves lightly breaking the outer coating with a nail file or similar tool to allow moisture to penetrate it.
Other species require cold stratification. This means they need a certain amount of time under cold temperatures to germinate. Place seeds in a bag with soilless materials like sawdust or coco coir, and moisten the bag with a few sprays of water. Place your seeds in the bag in the refrigerator for as many days as they need.
Premoistened soil makes planting easier than usual. Simply make little holes in the soil mix and sow seeds at their appropriate depth. Some plants, like chamomile, prefer surface sowing and need access to light to germinate. Others prefer deeper depths, while most crops prefer a depth of half to a quarter inch.
Tamp the soil after sowing seeds to ensure they stick to it. Cover the holes of deeply planted ones, and press surface sowings down so they touch the dirt. You want to press the soil down without compressing it. A gentle hand is key!
Step 5: Place Seedlings Under Light
Some seedlings need bright lights to grow their best, while others may burn under excessive direct sunlight or when they’re too close to grow lights. Give them the right light exposure and they’ll thank you with healthy growth.
It’s generally best to keep seedlings a foot or two away from grow lights. If using natural light, choose a site with bright indirect sunlight most of the day or between four and six hours of direct sunlight. Some plants may grow lopsided near a window; simply rotate them every week or two to ensure they grow equally on all sides.
Grow lights turn on and off when you choose to flip the switch. Place them on a mechanical or electronic timer for easy usage. Set them up with eight or more hours of “on” time and a corresponding amount of “off’ time.
Step 6: Use a Heat Mat (If Necessary)
Unless your heater is on all the time, your house may grow chilly during the winter! Although you can put on extra blankets and socks, your plants struggle with the cold temperatures. A seedling heat mat is a simple tool that boosts soil warmth and maintains it at a steady level.
A seedling heat mat isn’t necessary for seed starting, but it’s immensely helpful in moderating soil temperatures. I’ve had zero pepper seeds sprout one year, then a week after placing them on heat mats I watched them all germinate. Use one if you know your growing room will be cold during fall, winter, and early spring.
Heat mats boost soil warmth by about 10 to 20 degrees. Place a tray on the mat, then put the containers with seeds into the tray. The tray collects water so it doesn’t sit on your mat. Once your plants sprout a few leaves you can choose to turn off the mat and let the plants adapt to ambient temperatures.
Step 7: Thin Seedlings
A few plants, like beets, form clumps of seeds rather than individual ones. They’ll sprout multiple seedlings in a small area, thus requiring thinning. Other plants have tiny seeds that germinate near each other and also require thinning!
To thin seedlings, pinch them above their roots so the stem snaps off. Leave all but one plant remaining—it’ll thrive in its container as it grows roots without having to share space. Eat edible seedlings like beets, lettuce, and onions. They’re essentially microgreens! Compost non-edible thinnings from ornamental flowers.
Step 8: Transplant When Ready
After caring for your seedlings all season you’ll be ready to move them outdoors! Look to their packets for specific dates on when to move them outside. Frost-tolerant species transition best around your last average frost, between two weeks before and after the date. With heat-loving species, wait for a few weeks of frost-free weather before it’s safe to move them.
Greenhouses, cold frames, and row covers help you cheat the seasons! They can give you a month or more of a head start on the growing season. Transition seedlings into them when they’re warm enough for the tender plants. Your flowers, veggies, and herbs will harden off naturally and be ready to explode with growth when you plant them in the ground.
If hardening off seedlings outdoors without protection, place them outside during the day and indoors at night for a week or two to help them transition. They’ll appreciate a sight with dappled sunlight or partial shade outdoors while they acclimate. After hardening off, plant your crops where you’d like them to thrive for the growing season.
And that’s it! Repeat the process as needed whenever it’s time to grow more plants.
