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How do you start seeds in a wicking tray?
How do you start seeds in a wicking tray?

You start seed in a wicking tray, beginning with sifting out pH neutral gravel.

Written by Halle Brake
Updated over a week ago

The Vertical Food Blog welcomes some great tips from Rob Torcellini from Bigelow Brook Farms, whose innovation is apparent in his clever growing techniques. 

As with most businesses, we to try to reduce waste as much as possible. This ranges from cutting costs in materials to reducing the amount of labor to complete tasks. 

We have found that when seeding in rock wool cubes, achieving 100% germination is rare. This means that many cubes will go to waste, never being used. Some growers will over-seed each cube, then have to spend time pinching off the excess plants. Also, by using standard seeds instead of pelleted seeds (in rock wool cubes), you can reduce your overall seed costs. 

As part of our own cost cutting goals, we developed the GrowGrip, a reusable plant holder, for growing leafy greens. They eliminate the need for using disposable rock wool cubes or mixing potting soil, as well as removing the hassle of using bulky net pots.

Save space and money by seeding wicking trays

As a companion to using the GrowGrip, we recommend that our clients start their plant seedlings (typically lettuce) in a simple wicking tray. This allows for hundreds of plants to be started in a small wicking tray.

1) Sift out a pH neutral gravel

We start by using a pH neutral gravel. We use expanded shale since it is half the weight of regular gravel, although most gravels will work well. To obtain the proper consistency, it is run through a 3/8” screen made from standard hardware cloth, then the smaller fines are removed by screening through a regular window screen. 

It’s important to get the smaller fines removed so the final product isn’t able to compact easily. If the gravel is damp, it is best to rinse the gravel over the screen to wash the fines out.

I’ve tried regular “road sand” which is fairly coarse. It works okay, but doesn’t stay as damp as the expanded shale. The shale is ideal as it is porous and wicks and holds moisture. With sands (or crushed rock), the layer in the tray usually needs to be a bit thinner. We’re currently working with our shale provider to get us a sand material that could be resold primarily for this use.

2) Create an automatic watering bottle

A standard 1020 tray works well and will hold roughly 500-600 lettuce seedlings in a single tray. The tray should hold about 1.5” of gravel. To keep the gravel consistently moist, we like to take a plastic bottle and drill a hole through the side of the lid. 

Then fill the bottle with nutrient water and place it in the wicking tray so that the bottom of the tray maintains a thin layer of water. The gravel will wick up the water and keep the upper layer damp enough to germinate the seeds. Depending on the gravel type, it may take some experimenting with the thickness of the gravel, or depth of water.

3) Sprinkle seeds over tray

To plant lettuce seeds, we simply sprinkle the seeds over the gravel then gently smooth out the surface to slightly cover them. After a couple of days, you should see sprouts and they tray can be placed in direct sunlight. It’s important to make that sure there is always water in the bottle as the plants can quickly dry out when the reserve is empty.

4) Keep the tray uncovered and warm

For maximum germination, it is best to use a temperature controlled heating mat in cooler climates or to keep the tray out of direct sunlight in warmer climates until the plants have germinated.

We have found that using a cover over the seeds is troublesome as the humidity level in this small environment will quickly rise, causing some seeds to rot. Mildew can also form on the new tender leaves with the high humidity. If sunlight reaches a covered tray, the temperature will quickly rise and sterilize the seeds or kill the plants. It’s best to leave the plants uncovered, maintain the water reservoir, and allow fresh air around the plants.

5) Loosen seedlings and transplant to GrowGrips or ZipGrow Towers!

Once the plants are a couple of weeks old, the best plants can be sorted and put into the GrowGrip. We have also found that this seedling starting method also works well with the Bright Agrotech’s ZipGrow Towers, as you don’t have rock wool cubes or potting soil spread through the towers.

After about two weeks, the secondary leaves should be mature and the lettuce will have a strong enough root system to transplant. We just gently grab a clump of the greens with one hand, then run our fingers through the gravel to loosen it up and the group of lettuce will come right out. A slight shake and the majority of the gravel will fall off the roots with little damage. Since the roots are not too long, the plants can be separated and individually placed into the GrowGrip. We have also found that this is an excellent technique for transplanting seedlings into a ZipGrow Tower, or even transplanting into media beds or soil gardens.


Using this simple wicking tray helps to save growing space by having tight clusters of plants, saves money and the environment by using reusable gravel, and helps to maintain a clean system by not having rock wool or soil mixtures get into your hydroponic or aquaponic system.

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