Best Seed Starting Trays: Cell Sizes, Types and What to Buy

The best seed starting trays for a home garden are a sturdy plastic flat with drainage holes, a cell insert sized to the crop, a solid tray underneath for bottom watering and a clear dome for the first days. Cell size is the part that matters most. University of Maryland Extension recommends 3 to 6 square inches per plant for greens and 9 to 16 square inches for tomatoes and peppers, so one 72-cell insert cannot serve a whole garden well.

Best Seed Starting Trays by What You Are Growing

Most seed-starting kits combine the same few parts. The flat is the outer tray, and University of Maryland Extension gives its usual size as 10.5 by 21 inches and 2 inches deep. Cell packs, also sold as inserts, are joined plastic cells designed to drop into that flat, with individual cells from 1/2 inch to 4 inches across. Plug trays are the sturdy one-piece version, a flat divided into cells. Utah State University Extension notes that inserts go up to 288 cells but that the common home sizes hold 36, 48 or 72 cells per flat.

Tray type	How it works	Suits	Watch out for
Open flat	Sow thickly across the 2-inch-deep flat, then move seedlings to cells at the first true leaves	Onions, leeks, lettuce sown by the hundred	Roots tangle; transplanting takes time
Small-cell insert, 72 cells	One seedling per cell, about 3 square inches each	Lettuce, spinach, kale and other greens	Too small to hold tomatoes or peppers to planting size
Medium insert, 36 to 48 cells	About 4.5 to 6 square inches per cell	Broccoli, cabbage, cucumbers, squash	Fewer plants per shelf
Large cells or pots, 3 to 4 inches	9 to 16 square inches per plant	Tomatoes, peppers, eggplant	Only 10 to 21 plants per flat
Peat pots and pellets	Compressed peat; pellets expand when watered	A dozen seeds with no mix to buy	Single use
Soil blocks	Mix pressed into freestanding cubes with a blocker tool	Gardeners who want to avoid plastic cells	Needs a tool and a solid tray

The per-cell areas in the table come from dividing the flat. A flat of 10.5 by 21 inches has about 220 square inches of floor, so 72 cells get roughly 3 square inches each, 48 cells about 4.5 and 36 cells about 6. Whole 4-inch cells fit two across and five along, which is 10 per flat, and 3-inch cells fit three across and seven along, which is 21.

Cell Size: The Decision That Follows You All Season

University of Maryland Extension matches cell area to crop in three groups:

• Lettuce, beets, spinach, kale and other greens: 3 to 6 square inches.
• Squash, cucumber, broccoli and cabbage: 4 to 9 square inches.
• Tomato, eggplant and pepper: 9 to 16 square inches.

Set those against the insert sizes and the buying decision gets simple. A 72-cell insert sits at the bottom of the greens range and suits nothing larger. A 36-cell insert covers the top of the greens range and the lower half of the squash and brassica range. Tomatoes and peppers need a cell 3 to 4 inches square, which no common insert count provides, so there are two honest routes. Sow them in small cells and move each seedling to a larger pot shortly after the first true leaves appear, which is when Maryland says to pot up. Or sow directly into 3-inch to 4-inch cells and accept fewer plants per flat.

Time indoors pushes in the same direction. Maryland’s schedule has leafy greens indoors for only 2 to 4 weeks, cabbage, broccoli and cauliflower for 4 to 6 weeks, and cucumber, squash and melons for 3 to 5 weeks. Tomatoes take 5 to 6 weeks, and peppers, eggplant and onions 8 to 10 weeks. The crops that need the most root room are also the ones that occupy the shelf longest. The guide to when to start seeds indoors turns those week counts into calendar dates for your frost date.

Depth, Drainage and How Long the Plastic Lasts

Two requirements apply to every container, bought or improvised. University of Maryland Extension says almost any clean container will do provided it drains well and is at least 2 inches deep. University of Maine Cooperative Extension gives the range as 2 to 3.5 inches deep with adequate drainage holes. A yogurt cup with holes punched in the bottom meets both, and Iowa State University Extension lists milk cartons and paper cups the same way.

Durability is where purchased trays differ. Utah State notes that plastic pots, trays and inserts vary in thickness, that the majority are single-use products, and that some can be reused for several years. Thin inserts tend to split where you squeeze a cell to release the root ball. If you plan to sow every spring, a rigid plug tray or a heavy-walled insert costs more once and less over time.

Reused trays must be cleaned. Scrub out the old mix, then disinfect in one part chlorine bleach to nine parts water, the ratio given by both Maine and Iowa State. Maryland’s version, for trays that held diseased seedlings, is a soak of 1 to 2 minutes in 1 part bleach to 10 parts water followed by a rinse with clean water.

Bottom Trays, Domes and Heat

Buy one solid tray with no holes for every holed flat or insert. The pair makes bottom watering possible. Maine’s method is to set the seedling containers in a tray holding 1/2 inch of water for 20 minutes, then lift them out or pour off the rest. Maine also names overwatering as the most frequent mistake growers make and advises letting trays get almost dry before watering again. Constantly wet mix and still air favor damping-off, the fungal disease that rots young seedlings, and a small fan moving a gentle breeze over the tops lowers the risk.

A clear dome holds humidity and warmth over the mix until the seeds come up. It is a germination tool only. Iowa State says to remove the covering as soon as the seed germinates, and Maine sets the trigger at about 50 percent of the seeds sprouting, for both the cover and any bottom heat.

Heat mats are the optional part of a kit. Maine notes that inexpensive mats warm the growing medium to about 70 degrees F, and Maryland puts a good germinating temperature for most vegetable seeds at 65 to 75 degrees F. A room that already holds 70 degrees needs no mat. Maine’s crop table shows the exception to watch: lettuce germinates best in cooler soil, 50 to 65 degrees F, so keep lettuce trays off the mat. The companion guide to grow lights and heat mats covers thermostats and sizing.

Light Comes With the Tray

A tray is only as useful as the light above it, so plan both together. Maine places lights 2 to 4 inches from the seedling canopy and runs them 16 hours a day. Maryland gives 2 to 6 inches and 14 to 16 hours. Measure the shelf before buying trays, since every flat needs its 10.5 by 21 inches fully under the lamp. Capacity that cannot be lit produces leggy seedlings that are no use in the garden. For the fixture itself, see the comparison of grow lights for seedlings.

Picking Your First Setup

For a first season with a mixed vegetable garden, two flats cover most needs:

1. One flat with a 36-cell or 48-cell insert for broccoli, cabbage, cucumbers, squash and greens.
2. One flat of 3-inch to 4-inch cells or pots for tomatoes, peppers and eggplant, either sown directly or filled by potting up.
3. A solid tray under each for bottom watering.
4. One clear dome, moved from flat to flat as each crop is sown and removed at about 50 percent sprouting.

Fresh potting mix carries seedlings a long way in cells this size. Maryland notes that most mixes contain enough fertilizer to grow a seedling for at least 6 weeks. When plants reach transplant size, handle them by the leaves and not the stems, then harden them off before planting out. For a single crop worked through from sowing to potting up, follow the walkthrough for tomatoes from seed.

Quick Recap

• Match cell area to crop: 3 to 6 square inches for greens, 4 to 9 for squash and brassicas, 9 to 16 for tomatoes and peppers.
• A 72-cell insert gives about 3 square inches per cell and a 36-cell insert about 6.
• Every container needs drainage and at least 2 inches of depth.
• Pair each holed insert with a solid tray and bottom water in 1/2 inch of water for 20 minutes.
• Remove the dome at about 50 percent sprouting, and disinfect reused trays in 1 part bleach to 9 parts water.