Ready. Set. Go. Or Grow.

Growing seedlings indoors seems almost like a race. Of course, it has a staggered start, with onions already growing strongly and tomatoes not yet sown. Watering these seedlings is crucial: timely watering keeps them chugging along apace; two or three days of neglect might spell death.

A simple way to automatically water seedlings is to rely on the soil to draw water, as needed, via capillary action. And this is just how some seed propagators that you can make or buy do their job of watering. Soil watered by capillary action stays constantly moist rather than swinging between the extremes of having plant roots cry for air and then for water.

An Ancient Concept

Capillary watering is nothing new. It has a long history — possibly a few thousand years! An olla (pronounced oya) is an unglazed ceramic vessel traditionally used for cooking soups or stew, and also for watering plants when sunk into the ground near a plant and filled with water.

As the roots suck up moisture from surrounding ground, the drying soil replenishes its moisture by drawing water out, via capillary action, from the vessel. That is, as long as the soil isn’t bone dry, in which case the capillary connection from the olla to the soil to the root is broken.

Olla, with schematic of water in and out

Ollas are still available for capillary watering, for art, and, of course, for cooking stews and soups. For irrigation, ollas are bulbous with a narrow fill spout that’s capped each time after filling. The gardeners two jobs with ollas are, first, to make sure there’s water within and, second, not to leave them out in freezing weather. They’ll crack.

A Modern Wrinkle

I’ve watered potted plants, mostly houseplants, with a couple of modern wrinkles on the traditional olla. These are hollow terra cotta spikes about four inches long and an inch across, with a watertight lid out of which is a flexible, plastic tube. After filling the spike with water and capping it, the far end of the tube is immersed in a large container of water.

Then the spike is pushed into the soil of the potted plant. As long as the soil starts out moist and there is a continuous water connection from the water into the spike and along the plastic tube into the water reservoir, moisture will be continuously replenished in the soil in synch with its removal.

I’ve also made my own olla, in my case for keeping a potting mix moist for rooting stem cuttings. I plugged the bottom of a terra cotta flower pot with a cork or some malleable wax left over from my daughter’s orthodontic experience. Sunk into the rooting mix in the center of a larger pot, and filled with water, that small pot kept the ring of rooting mix moist for the cuttings I stuck there. I covered the small pot to reduce water loss from evaporation.

Many Plants, Small Space, Easy Watering

Back to the current seedling race, watering them. For years, capillary matting — a thick, water absorbent fabric that does not rot — has been available, mostly to commercial greenhouse growers. The idea is to let one end of the fabric dip down into a water reservoir while the remainder of it rests flat on a horizontal surface. 

Pots of plants sit on the flat mat. If the pots likewise have flat bottoms and the soil within is right up against the bottom of the pots, a capillary water connection is established throughout. As plants drink in water from the potting mix, soil moisture is replenished by water drawn up from the mat which, in turn, draws it up from the reservoir.

The capillary watering systems that you can buy are nifty setups that make it very convenient for raising seedlings in your home. A plastic pan holds water. Into the pan fits a plastic “table” on which sits the mat, with one of its ends dipping into the water. A multi-celled planting tray sits atop the mat.

The whole setup is about the size of a three-ring notebook, or half that, depending on the number and size of the cells. Right now, lettuce — twenty four seedlings in only about a square foot of space — are growing in one-inch cells. In about a month, I’ll have cucumbers and melons started in two-inch cells. Obviously, a single capillary watering system is not enough for all the seedlings of most gardeners.

As a plant grows, its roots knit together the soil in its cell. When it’s time to transplant, I just lift the plant out of its cell, roots intact and hardly disturbed. And after a good washdown, the whole setup is ready for re-use year after year.

Lifting sprouts for transfer to individual cells

Some Buts

Capillary watering does have a few drawbacks. Water evaporating at the surface of the soil leaves mineral residues which can accumulate and draw water out of the roots in the same way that potato chips can dry out your lips. Seedling generally don’t spend enough time in containers to bring on this problem, especially when care is taken not to overfertilize. If necessary, periodic watering from above can wash the salts away through the soil.

Another problem is that of seedling roots growing out through the bottom of their cells into the matting. Then plants become difficult to remove from their cells and lose too many roots when they are finally ripped away. I avert this problem by periodically lifting the planting tray up off the mat beginning when the seedlings are large enough for their roots to bind the soil together. Or, if the roots have already grown down into the matting (some kinds of mats keep roots from doing this), I slice them off with a long, thin knife — actually, an old bread knife — right at the bottom of the cell tray.

Because the watering is automatic, or almost, the worst threat, perhaps, to any automated system is neglect. I have almost lost seedlings from forgetting to check the water level, which only needs to be done about weekly, more or less, in the reservoir.

Despite these minor shortcomings, capillary watering systems for seedlings are among the few horticultural gizmos that I wholeheartedly recommend. Before I used them, my seedlings tethered me to my house daily throughout April and May as tenaciously as milch cows tether the dairy farmer to the barn.