How to Start Seeds Indoors
Starting seeds indoors gives your garden a head start, letting you grow varieties that never show up as transplants at the store while saving money over buying flats each spring. With a sunny window or an inexpensive light, a few clean containers, and the right timing, most home gardeners can raise sturdy seedlings that transplant well once the weather settles.
Start by working backward from your average last spring frost date, which your local Cooperative Extension office or a frost-date lookup can give you for your zip code. Most seed packets tell you how many weeks ahead to sow, and the common rules of thumb are roughly six to eight weeks before the last frost for tomatoes, peppers, and eggplant, and about three to four weeks for fast growers like squash and cucumbers. Sowing too early is the most common beginner mistake, because seedlings held indoors too long get tall, weak, and rootbound before it is safe to plant them out.
Use a light, fluffy seed-starting mix rather than garden soil or heavy potting soil. A true seed-starting mix is sterile and drains freely, which cuts down on the fungal problems that make seedlings flop over at the soil line. Moisten the mix in a tub or bucket until it feels like a wrung-out sponge before you fill your containers, since dry peat and coir shed water and are hard to wet evenly once packed into cells.
Almost any small container works as long as it has drainage holes: cell trays, recycled six-packs, yogurt cups with holes poked in the bottom, or peat pots. Fill them, firm the mix gently, and sow at the depth the packet lists, which is usually two to three times the seed's diameter. Very fine seeds and a few kinds that need light to germinate are just pressed onto the surface, so check the packet before you bury anything, then label every container right away because seedlings look nearly identical for the first couple of weeks.
Warmth drives germination more than light does at this stage. Most warm-season vegetables sprout fastest with the mix in the low-to-mid seventies Fahrenheit, so a spot on top of the refrigerator or a seedling heat mat speeds things up. Cover the trays loosely with a clear dome or plastic wrap to hold humidity until the first sprouts appear, then take the cover off promptly so air can move and the surface can dry slightly between waterings.
The moment seedlings break the surface, they need bright light, and this is where a windowsill usually falls short. A south-facing window can work in a pinch, but seedlings reaching and leaning toward the glass are telling you the light is too weak. An inexpensive shop light or LED grow light hung a few inches above the leaves and run about fourteen to sixteen hours a day, on a simple timer, produces much stockier plants. Water from the bottom when you can by setting trays in a shallow tray of water, let them soak it up, and pour off what is left so roots are never sitting in standing water.
Before transplanting outdoors, harden off the seedlings over roughly a week to ten days. Set them in a sheltered, shaded outdoor spot for an hour or two the first day and gradually give them more time, more sun, and more wind each day so they toughen up. Skipping this step shocks tender indoor-grown plants, and a warm afternoon of full sun can scorch leaves that were never exposed to it. Once nights stay reliably above your crop's comfort range and the plants are hardened off, move them into the garden.
Frequently asked questions
Why are my seedlings tall, thin, and falling over?
That stretched, leggy look almost always means not enough light. Seedlings on a windowsill reach toward the weak light and grow spindly, so move them under a grow light kept just a few inches above the leaves for fourteen to sixteen hours a day. A small fan for gentle air movement and slightly cooler temperatures after germination also help produce shorter, stockier stems.
Do I need a heat mat and grow lights, or can I skip them?
You can start seeds without either, but results are more reliable with them. A heat mat mainly speeds up germination for warm-season crops, so if your home is warm you may not need one. A light is harder to skip once seeds sprout: unless you have a genuinely bright south-facing window, an inexpensive shop light or LED makes the difference between sturdy transplants and weak, stretched ones.
How do I keep seedlings from damping off and dying at the soil line?
Damping off is a fungal problem, and prevention is the whole game since there is no cure once a stem collapses. Use a clean, sterile seed-starting mix and clean containers, avoid overwatering by letting the surface dry slightly between waterings, and give the seedlings good air circulation with the humidity dome off and a gentle fan. Watering from the bottom and not crowding the cells also go a long way.
General guidance for US home gardens. Conditions vary by region and season; check with your local Cooperative Extension office for site-specific advice. Page updated Aug 4, 2026.