Best Self-Watering Seed Starter Trays

Why seedlings are where self-watering makes the most sense

Elsewhere on this site we tell you that a reservoir pot is the wrong tool for a seedling, and we stand by it: a reservoir delivers water to a zone, and a seed tray is not a zone. A self-watering seed starter is a different design. There is no deep tank and no wick into a single pot. It is a shallow reservoir, a felt capillary mat, and a tray of small cells standing on the mat, each cell drawing water up through its drainage hole.

That design suits the job. University of Maryland Extension puts it plainly: seed germination begins with water absorption, and an adequate and continuous supply of moisture is essential. Clemson Extension adds that seeds are extremely sensitive to drying out. And a seedling cell holds very little mix - the GrowEase’s published 3 quarts is shared across 24 cells, half a cup each. NC State Extension’s container chapter notes that larger containers do not dry out as quickly and need less frequent watering, which is the same point from the other end.

Bottom watering is also one of the two methods UMD describes for seedlings, and Clemson Extension gives the manual version - stand the tray in 1 to 2 inches of water for 3 to 10 minutes, then lift it back out. A capillary mat does the same thing continuously, which is the greenhouse technique the mat was invented for. The page on bottom watering covers the manual method for houseplants in more detail.

The risk it introduces: mix that never gets a chance to dry

Continuous is also the problem. UMD says the common problems in seedling care come down to root moisture - a lack of uniform watering, overwatering, and letting the containers dry out - and warns that the water molds Pythium, Phytophthora and Rhizoctonia cause root rot and damping-off that can quickly kill a whole container of seedlings if the mix is kept too wet. A mat is designed to fix the first and the third. Left brim-full with a dome on top, it can create the second.

It is the same mechanism we set out for root rot in self-watering pots, at a smaller scale and with less margin. Three habits keep it under control, and each rests on published guidance - two from extension services, one from a maker - rather than on our say-so.

Take the dome off on time. Clemson says to keep a germination dome on until all the seeds have germinated; University of Minnesota Extension says to remove it once seedlings are tall enough to touch it. Either way, a dome is for germination, not for growing on.

Do not fill past the maker’s line. SOLIGT is the only maker here that publishes a fill level - about 3/4 inch - and it is a useful habit with any tray. The reservoir is there to keep the mat wet, not to flood the cells.

Use a seed-starting mix, not garden soil. Clemson notes that soilless mixes reduce soil-borne diseases such as damping-off. The mixes are ranked here, including which ones re-wet properly on a capillary mat.

How we ranked them

We have not grown seedlings in any of these kits. There is no germination trial here and no side-by-side shelf. What follows is each maker’s published specification, checked against published extension guidance, with our arithmetic shown where we did any.

Published cell size first. Cell size decides what a tray can grow and for how long, and UMD publishes a recommended range for each group of crops. A tray whose maker prints the cell dimensions can be checked against it before you buy.

Published reservoir capacity second. It is the one number that makes the self-watering claim checkable, and only one maker here prints it.

Moisture control third. A published fill level, a visible water line, a platform that keeps cells out of the water - design features that make the overwatering risk above easier to manage.

Light, where a kit includes one. Measured against the hours three extension services recommend, because a light that cannot run long enough is a light you still have to supplement.

Cells per kit and value last. A cheap cell that is too small for your crop is not good value.

Cell size, crop by crop

University of Maryland Extension publishes recommended individual cell sizes by crop group. Put the trays against them and the picture is clearer than any listing makes it.

Two things follow. The SOLIGT’s 2.25 square inch cells are below the smallest range UMD gives for anything, and the BlumWay does not publish a cell width at all, so neither can be placed on this table.

And no self-watering kit we found reaches the tomato range. That is not a reason to avoid them, because the extension guidance does not expect a tomato to finish in its first cell. UMD says tomatoes, peppers and eggplant are often started singly and moved into larger containers 2 to 3 weeks after germination. Clemson gives one or two sets of true leaves as the moment to transplant into larger containers, and UMN recommends a new container 1 to 2 inches wider than the cell. A small cell is a schedule, not a failure - as long as you know it is coming.

It is also worth knowing what not to start in a tray at all. Clemson lists beans, corn and carrots as crops that do not do well as transplants and should be sown directly in the garden.

Depth is not area

Burpee’s listing describes its cells as ideal for deep-rooted plants like tomatoes, peppers and eggplants. Its published cell is 2 1/4 inches square and 2 inches deep. That is 5.06 square inches - the largest cell area on this page, and still only a little over half of the 9 square inch minimum UMD recommends for exactly those crops. The depth, meanwhile, is precisely the 2-inch minimum UMD gives for any seed-starting container.

The Gardener’s Best XL is the genuinely deep one, at 3 1/4 inches, with vents for air pruning and a pop-out hole in each cell base. Its cell area is 5 square inches, the same as the Burpee in practice. So the XL buys you a longer stay in the tray before roots run out of room downward; it does not buy you the width UMD recommends for fruiting crops. At the other end, the BlumWay’s published 1.97-inch depth is a hair under UMD’s 2-inch minimum.

The one number most makers leave out

Every kit here is sold as self-watering. Gardener’s Supply is the only maker that tells you how much water its reservoir holds: 10 cups on the 24-cell GrowEase, 5 cups on the 12-cell GrowEase, and 5 cups on the Best XL. Burpee, SOLIGT and BlumWay describe a water tray and a mat and stop there.

The GrowEase also publishes its mix capacity, which lets you check the ratio. Ten cups of water under 3 quarts - 12 cups - of mix is 0.83 cups of reservoir per cup of mix. The 12-cell version holds 5 cups under 1 1/2 quarts, which is the same 0.83. Gardener’s Supply scales the reservoir with the tray, the same pattern we found on its elevated raised beds.

BlumWay goes further than anyone in the other direction. Its listing says one watering is enough to take seeds from germination to strong seedlings, and it publishes no reservoir volume to back that up. We cannot check the claim, so we do not repeat it as a feature.

Why there is no refill interval on this page

Everywhere else on this site we divide a published reservoir by the volume of one thorough watering and multiply by the watering cadence Virginia Cooperative Extension publishes for container houseplants. The method is set out here.

Seedlings are outside that method. None of the extension guidance we rely on publishes a watering cadence for seedlings on a capillary mat, and a tray’s draw changes as the seedlings grow. Clemson recommends a fan once seedlings have two true leaves, and notes that it raises transpiration and dries them out, so moisture management matters more from that point. A number here would look precise and mean nothing, so we check the reservoir the way you should: look at it every day.

Light, which no tray solves on its own

A self-watering tray takes care of water. It does nothing for the other thing seedlings indoors most often lack. University of Minnesota Extension calls artificial light essential for healthy seedlings and gives 12 to 16 hours a day, with the light no more than 4 inches above the seedlings. UMD recommends 14 to 16 hours with the tops 1 to 2 inches from the lights. Clemson recommends 16 to 18 hours and warns that windowsill transplants reach for the light and become tall and leggy.

That is the problem with the BlumWay, the one kit here sold with a light. Its longest timer setting is 12 hours - the floor of Minnesota’s range and short of the other two - and BlumWay publishes no light output figure, so there is no way to judge whether eight LEDs over a tray are enough at any duration. Treat it as a supplement to a bright window, not as the light.

Converting a tray you already own

If you already have ordinary cell trays and a flat, you do not need a kit. A sheet of capillary matting in a flat without holes, with the cells standing on it, is the same system. UMD describes the standard 10 x 20 inch flat, notes that flats without holes act as a saucer, and warns that care must be taken to avoid overwatering undrained flats - which is the same fill-level discipline as above.

The one requirement you cannot skip: UMN says every seed-starting container must have drainage holes in the bottom. For a mat, those holes are also the only way water gets in.

Also considered

Jiffy Self-Watering Seed Starter Tray Kit, 4 pack. Four 34-cell trays, 136 peat pellets and a wicking mat. Jiffy publishes a pellet diameter rather than a cell size, and no reservoir figure, so there is nothing on it to rank.

Gardener’s Supply Galvanized Steel Seed Starting Tray, 24 cell. A steel tray with a wire grid over the water and a compostable jute mat. It publishes its soil capacity, about 19 cups, but not the reservoir, and it costs considerably more than the plastic GrowEase for the same cell count.

Hanaoyo Self-Watering Seed Starter Tray, 2 packs. 96 cells with 4.1-inch domes, at a low price, but no published cell dimensions at all.

Burpee 72-Cell Self-Watering Seed Starter Kit. Listed as out of stock when we checked, so we have not linked it.

The honest cons on every pick

GrowEase 24-cell. Twenty-four cells is a small sowing. Its cells are under half the area UMD recommends for tomatoes and peppers. The dome is a low 2 3/4 inches. It costs more per cell than the Burpee or the SOLIGT.

Gardener’s Best XL. Twelve cells, by a distance the most expensive per cell here, and the humidity domes are sold separately. Deep cells, but not wide ones, and no published mix capacity.

Burpee XL. No published reservoir figure. The tomato claim in the listing outruns the cell size. The 32 coir pellets cover one sowing of 32 cells, so the next one needs pellets or loose mix.

SOLIGT 6 pack. The smallest cells here, below every UMD range, so everything moves on early. Reservoir volume is not published. Six of everything to fill, check and wash, and the highest total price on the page.

BlumWay 5 pack. A 12-hour timer, an unrated light, no published cell width, a depth a hair under UMD’s minimum, and a one-watering claim with no reservoir figure behind it. Cheap, and priced accordingly.

What we would buy

The GrowEase 24-cell for a first self-watering tray, because it is the only kit where the self-watering claim comes with numbers attached and its cells suit the greens, herbs and brassicas most people start first. The Burpee XL if you want more cell area for less money and can live without a reservoir figure. The Gardener’s Best XL only for a handful of tomatoes or peppers you want to keep in the tray a little longer.

And once they are big enough to plant out, the sub-irrigated vegetable and herb planters are where the reservoir arithmetic starts to apply again.