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Beginner’s Guide: How to Start Seeds Indoors

By: Angela Judd | Updated: May 11, 2026

Starting seeds indoors is a skill, and like any skill, it takes practice. I’ve started thousands of plants from seed, and I’ve lost my share along the way. These are the ten steps I follow now, developed over years of starting seeds in a hot, dry climate where the timing and temperatures don’t always align with most seed-starting advice.

A person planting seeds in soil trays, holding seeds in one hand over a black grid table.

Key Takeaways

Starting seeds indoors requires practice and the right conditions, like maintaining a temperature of 70-80°F.
Use good supplies, including a seed-starting mix, containers with drainage, and adequate lighting to support seedling growth.
Moisten the seed mix properly before planting and ensure proper planting depth and spacing for seeds.
Care for seedlings by providing sufficient light and water, avoiding waterlogged soil to prevent root rot.
Harden off seedlings gradually before transplanting outdoors to reduce transplant shock.

Article Outline:

The right temperature and location
The supplies I use
Setting up the space
Organizing seeds and making labels
Moistening the mix and filling containers
Planting the seeds
Caring for seedlings: light and water
Thinning to the strongest seedling
When to start feeding seedlings
Hardening off and transplanting
FAQ’s about Starting Seeds Indoors

The right temperature and location

Most seeds germinate best when the seed-starting mix is between 65°F and 80°F, depending on the crop. Cool-season crops like broccoli can handle the lower end of that range, while warm-season crops like tomatoes and peppers germinate and grow much better when the mix is closer to 75°F or warmer.

Two indoor plant shelves with various potted plants and seedlings near a window and under grow lights.

Most seedlings grow best when the air temperature is between about 65°F and 75°F. A little warmer is usually fine for warm-season crops, but once the space is consistently above 85°F, it may be too hot for good germination and healthy seedling growth.

In the low desert, the garage is usually too hot for seed starting in summer. Even if the house feels cool, the garage can be much hotter. I In winter, the opposite can happen, and I use a heat mat when the indoor air isn’t warm enough. A few crops prefer cooler temperatures to sprout. Lettuce and onions usually do best closer to 60°F to 70°F. Use this seed germination temperature guide to check the range for what you’re starting.

When seeds have moisture but still aren’t coming up, temperature is the first thing I check. I’ve had trays sit and do nothing for days, and more than once, the soil was simply too cool.

Timing is another important part of seed starting. This post walks you through how to start seeds indoors, but if you’re gardening in the low desert and wondering when to start each crop,see my guide on when to start seeds indoors.

The supplies I use

Good tools save time and cut down on frustration. You don’t need much to start, and you don’t need anything fancy. Fresh seed-starting mix, containers with drainage, a light, and labels will get you strong seedlings. For the specific trays, lights, and mix I reach for, see my favorite garden supplies.

Tomato seedlings growing in black pots on a metal shelf under indoor grow lights, with plant labels visible.

Essential Supplies for Starting Seeds Indoors:

Seeds: For trusted varieties that grow well season after season, visit my favorite seeds.
Seedling Pots and Tray: Use what you have on hand. I love these trays from Burpee and these from Bootstrap Farmer.
Seed Starting Mix: Use a sterile, bagged mix. Don’t use regular potting soil. I like this one.
Light Source: LED lights work well. Choose one that fits your space and budget. The light from a window is insufficient to grow vigorous, healthy seedlings indoors. See one I like here.

Nice-To-Have Indoor Seed Starting Supplies:

Humidity Domes: Good-quality domes will last for many seasons. You can also cover seedlings with plastic wrap or burlap to retain moisture.
Vermiculite: It allows light to penetrate the surface of sown seeds while keeping the soil hydrated. It also helps prevent algae from forming and fungus gnats.
Heat Mat: Keeps the soil at the optimal temperature of 70-80°F for germination.
Labels: These should be small enough to fit inside the humidity dome. Get creative and use what you have on hand. Wood labels may bleed. These labels give you plenty of room to write. .
Fan: This is the one I use. It ensures air circulation, which helps seedlings grow strong and prevents disease.
Power Strip and Timer: Use one with a timer for your lights and another for the heat mats and fans.
Watering Can: A long spout makes indoor watering easier.
Wire Shelving: Ensure the dimensions fit your trays and lights.

Setting up the space

Get the growing area ready before you plant. Set up your lights, plug in the heat mat, and check that everything fits and works. Clean and sanitize your seed pots and trays so you don’t carry last season’s problems into this one.

Organizing seeds and making labels

Packets of seeds and yellow plant tags labeled with vegetable names spread out on a wooden table.

Sort your seeds, decide how many of each to plant, and make your labels before your hands are in the soil. It’s a small step, but the process goes much more smoothly when I do this before I start.

If possible, I like to group crops by how quickly they germinate. Quick sprouters go in one tray, slower ones in another. If you’re not sure how long seeds take to germinate, check the seed packet.

Grouping similar seeds together will usually get you pretty close. Seed size can often give you a clue about germination time, but the packet is the best guide. When fast and slow seeds share a tray, you may have to take the humidity dome off too early for some seeds and too late for others.

Moistening the mix and filling containers

Getting the seed-starting mix to the right moisture level can take a little time. Sometimes it looks wet enough on the surface, but there are still dry pockets inside. Give the mix time to absorb the water.

I do this one of two ways. Sometimes I moisten the whole bag or tub of mix first, then fill the containers. Other times I fill the containers with dry mix, water from the top, fill the tray below with water, and let everything sit for a few hours or overnight before I plant. The second way keeps my hands a little cleaner and takes a little longer.

Either way, you want the mix damp all the way through, but not soggy. Fill the containers all the way up, firm the mix gently, and make sure there aren’t large air pockets before you plant.

Three-panel image showing soil being watered, scooped, and mixed by hand in a large metal container.

Planting the seeds

Follow the seed packet for planting depth and spacing. I usually plant 2 to 3 seeds per pot to improve the odds that at least one comes up. Press the seeds gently into the mix for good contact, then cover them to the depth listed on the packet. Some seeds need light to germinate, so don’t bury those unless the packet says to.

If your seeds are a few years old, sow a little more thickly. Germination rates drop over time, though seeds often last longer than people think. A few extra seeds per pot can help make up for lower germination.

After planting, sprinkle a layer of vermiculite over the mix. Then cover the tray to hold moisture in until the seeds germinate. A humidity dome works well. If you don’t have one, plastic wrap over the top can do the same job. When the mix is well hydrated before planting and the dome stays on, there is usually enough moisture for the seeds to germinate without watering again. You can mist the top of the pots with a spray bottle too if you feel like they are drying out.

Three panels: pouring seeds into a hand, planting seeds in soil, and holding a bucket of seeds over seed trays.

Caring for seedlings: light and water

Take the dome off as soon as the seeds are up. I once left town with the dome still on a tray that had already germinated. The seedlings sat in that trapped humidity for days and struggled to get even light, and the plants struggled.

Seedlings need bright light close overhead the moment they emerge. Set it too high, or give them too little, and they stretch tall and thin, reaching for it. My first year I started seeds in a window with no grow light. They grew long and floppy, and I didn’t know that was a problem; I didn’t realize they weren’t supposed to look like that. Leggy seedlings are weak seedlings. A grow light kept close prevents it.

Young seedlings with thin stems and small green leaves growing in dark, moist soil.

Once I got a light I thought more light meant faster growth. But, seedlings need a stretch of darkness each day too. Mine run on a timer now. Direct lighting for your growing seedlings is recommended for 14-16 hours daily.

You’ll need to start watering your seedlings. You want the soil hydrated, never sitting in water. When water pools in the tray and the mix stays soaked, roots rot. So I water deeply, then pour off whatever collects in the bottom. If it’s hard to pour, I pull the extra out with a syringe.

To tell when they need water, I touch the top of the mix and lift a pot to feel its weight and check the sides. When the soil starts pulling away from the inside of the pot, I’ve waited a little too long.

Wet soil is also what brings fungus gnats. Watering from the bottom helps, since the surface stays drier, and a thin layer of vermiculite on top helps too.

Seedlings growing under fluorescent lights on the left; hand tending seed trays indoors on the right.

Thinning to the strongest seedling

When several come up in one pot, thin to the strongest one, and do it early. The sooner you thin, the faster the keeper sizes up, because it isn’t competing for light, water, and root room. While seedlings are tiny and still have only their first leaves, you can pull the extras straight out. Once they’re bigger and rooted in, snip them at the soil line instead, so you don’t tear the roots of the one you’re keeping. If you’d rather save the extras, pot them up into their own containers.

When one pot comes up empty and the pot next to it has two seedlings, I fix it with a pencil. I slide the pencil under both, lift them together, gently tease them apart, and settle one into the empty pot. It saves the spare seedling and fills the gap.

Left: Hand planting seedling in tray. Right: Young green plants growing in soil in a seed tray.

When to start feeding seedlings

Seed-starting mix doesn’t hold much in the way of nutrients, so seedlings need feeding once they’re growing. Start a diluted fertilizer after the first set of true leaves appears. The true leaves are the second set, the ones that look like the plant, not the first rounded pair.

Begin by adding a half-dose of seaweed fertilizer to your water. One week later, feed with 1/2 dose of fish fertilizer every other time you water.

Left: Hand pours liquid from jug. Right: Watering can waters small potted plants on a black table outdoors.

Hardening off and transplanting

Once your seedlings have several sets of true leaves and the outdoor soil is at the right temperature, they’re close to ready. Don’t move them straight from the shelf to the garden. Hardening off first keeps them from getting stressed or dying.

About a week out, turn the heat mats off during the day and leave them on only at night. Then start setting the seedlings outside in a shady, protected spot for an hour or two, adding more time and more sun each day. You can learn more in this guide: “How to Harden Off Seedlings.”

Transplant timing matters here in the low desert. For spring planting, transplant in the morning so plants have time to settle in before nightfall. For fall or monsoon planting, transplant in the early evening so they can adjust as temperatures drop overnight rather than during the heat of the day.

Seedlings and young plants growing in trays and small pots, labeled and arranged on a table in sunlight.

I try to start seeds at the beginning of each planting window when possible because it gives me stronger, larger transplants. A bigger transplant has a bigger root ball. It holds moisture longer and gives you a little more room for error when the weather turns.

A couple of years ago, we had an intense October heatwave. My small snapdragon transplants and plugs died, but the larger transplants I had grown out came through fine. That experience reminded me how much transplant size can matter when it gets hot.

Knowing exactly when to start seeds indoors can be tricky sometimes. If your timing is off and your seedlings outgrow their containers or roots start coming through the drainage holes, you need to pot them up into larger containers. This step may not always be necessary, depending on the size of your seedlings and their original containers.

When I’m ready to plant outside, I make a label for each one. This is how I label my plants so I’m not guessing later which variety is which.

Small labeled pots of herbs and vegetables growing in a rustic wooden box in a sunny garden.

FAQ’s about Starting Seeds Indoors

What temperature do seeds need to germinate indoors?

Most seeds germinate best between 70 and 80 degrees. A few cool-season crops, like lettuce and onions, prefer it cooler, closer to 60 to 70. If your space runs colder than the seeds want, a heat mat brings the soil temperature up.

How many seeds should I plant per pot?

I plant 2 to 3 seeds per pot. Not every seed germinates, so a few per pot improves the odds of getting at least one strong seedling. Once they’re up, thin to the strongest one.

Do I need a heat mat to start seeds indoors?

It depends on your space and the season. In winter, when indoor air is cool, a heat mat helps seeds germinate faster and more reliably. In summer here in the low desert, indoor temperatures are usually warm enough on their own.

Why are my seedlings tall and leggy?

Leggy seedlings are almost always a light problem. When light is too weak or set too far above the seedlings, they stretch toward it and grow thin and floppy. Keep a bright light close overhead, and lower it or move it closer as soon as you see stretching.

When should I start fertilizing seedlings?

Start a diluted fertilizer once the first set of true leaves appears. True leaves are the second set, the ones shaped like the mature plant. The first rounded pair are seed leaves and don’t need feeding.

Why aren’t my seeds sprouting?

If the seeds have moisture and enough time and still nothing’s happening, check the temperature. Cool soil is the reason I most often find behind a tray that won’t come up. Old seeds can also germinate slowly and unevenly, so if yours are a few years old, give them extra time and sow a few more per pot.

How do I know when to water my seedlings?

Touch the top of the mix, and lift a pot to feel how heavy it is. You want it hydrated, never sitting in water. If the soil starts pulling away from the sides of the pot, it’s gone too dry. If water pools in the bottom of the tray, pour it off so the roots don’t rot.