How to Use a Seed Starting Tray? | From Cells to Garden

A seed starting tray works by filling cells with a sterile mix, sowing seeds, and keeping them warm and moist until they sprout.

Starting your own seeds is one of the cheapest ways to fill a garden, and the 1020 tray system is the workhorse behind it all. Whether you bought a 72-cell kit or a basic flat, the process stays the same: damp mix, correct depth, steady moisture, and light the moment those first true leaves appear.

Preparing The Tray Mix

Fill each cell with a lightweight seed-starting mix, which is commonly made of peat moss, vermiculite, and perlite. Regular potting soil or garden dirt is too dense for tray cells and can harbor diseases, so stick with a dedicated mix.

Moisten the mix before you fill the tray — it should feel like a wrung-out sponge. Loosely fill each cell without packing it down; compacted mix suffocates the seed and slows germination. Leave the surface slightly below the rim so water doesn’t spill between cells.

How Deep Do You Plant Seeds In A Tray?

Plant seeds about two to three times their diameter, but the seed packet overrides that rule every time. Some seeds, like lettuce, need light to germinate and should sit on the surface, while others want a quarter-inch or more of cover.

After sowing, mist the surface gently or set the tray in a shallow pan of water so the mix pulls moisture up from below. Place the humidity dome on if your kit includes one, and keep the tray somewhere warm — most common garden seeds germinate well between 65 and 75°F.

Once sprouts appear, remove the dome promptly. Leaving it on too long traps excess humidity, which encourages mold and weak, leggy growth. Move the tray to a bright windowsill or under grow lights immediately, because seedlings stretch toward any light they can find.

Watering And Hardening Off Seedlings

Before germination, gentle top-watering or misting keeps the mix evenly moist without disturbing seeds. After sprouting, switch to bottom-watering: pour water into the tray’s base and let the cells soak it up through their drainage holes.

About a week before transplanting, start hardening off the seedlings. One reliable method is moving them to a protected, shaded outdoor spot for two to three hours daily, gradually increasing their exposure over five to seven days. This acclimates them to sun and wind, so the shock of transplant is far less severe.

A Quick Note On Waterlogged Mix

Consistently moist is the goal; waterlogged is the enemy. Excess moisture in tray cells encourages mold on the surface and rot at the roots, both of which end the run quickly. If your tray has drainage holes, never let water pool above the cell bases themselves.

Label Everything Before You Forget

Seedlings of different varieties look almost identical in their first weeks. Write the name and sowing date on a plastic label as soon as you plant each cell — future you will be grateful when 72 identical green sprouts stare back.

When You’re Ready To Buy

A 72-cell tray is the most popular starting point because it fits a standard 1020 flat and holds enough variety for a home garden. If you’re shopping for your first kit, our tested roundup of the best 72 cell seed trays compares the dome options and drainage designs that matter.

With your tray filled, seeds sown, and the dome popped on, the only thing left is patience. Check the mix daily, lift the dome when green appears, and start the hardening-off clock about a week before the last frost date.