All it takes is just a little water and fertilizer to grow a head of lettuce with a low-cost, efficient system of hydroponic gardening widely known as “the Kratky method.”

“We can grow a head of lettuce on a gallon of water,” said professor Bernard Kratky, who began developing the method in 1987, experimenting with the simplest of equipment —“I grew beautiful cucumbers in soda cans.”

The researcher emeritus at the University of Hawaii at Manoa’s College of Tropical Agriculture and Human Resources has retired to Washington state, where he enjoys watching his vegetables grow every day at home using his brand of hydroponics.

“It’s adaptable to your needs and ability,” Kratky said of the system, whether you’re a backyard gardener or a commercial grower. He designed it to remedy the problems that plague Hawaii farmers: nematodes and other soil diseases, the heavy use of pesticides, and the high cost of water and electricity.

When he started his experiments, he rooted cucumber seedlings in 12-ounce cans, and the plants grew to extend several feet beyond the can, Kratky said, showing off some old photos. “You may be familiar with the saying: If can, can; if no can, no can. We had a lot of fun with that by changing it to: Can grow in cans; no can grow, if no can.”

Jensen Uyeda, a CTAHR extension agent, concurs that the Kratky method is the simplest and least expensive system to set up because there are fewer moving parts and no need for electricity to run a circulatory pump.

“You can forget about it and harvest in a month,” he said. “It’s the most productive in my opinion.”

For the home gardener, the method requires minimal knowledge, few supplies and takes up little space, easily fitting onto a condominium balcony or a small patio. It’s easy to maintain as long as the system is set up with the correct specifications, he said.

Uyeda has held public workshops on the topic for the past 12 years at the Urban Garden Center in Pearl City, but his main role is as a consultant to commercial farmers on all agricultural issues. Most of the 15 hydroponic farmers statewide use circulatory pumps because they have larger systems to maintain, but he knows of one small farmer who has been using the Kratky method quite successfully.

It just requires a bucket or tank (the container should not be transparent to discourage algae growth), plastic net pots, water and fertilizer. Larger systems require a piece of plastic foam to float on the water or a piece of wood with holes cut in it from which plants are suspended.

Crops that do well include lettuce, cabbage, green onions and cilantro — common ingredients in everyday cooking that have short growing cycles. The system can be modified to grow tomatoes and eggplant, which take longer, but vegetables with more extended growth cycles don’t usually do well, Uyeda said.

The big advantage of growing vegetables without soil is that it keeps your hands cleaner. There’s still an insect problem, mainly mosquitoes, but that can be mitigated with repellent tabs that dissolve in water or with the use of guppies and similar fish, he said. Snails and slugs, which bring the danger of rat lungworm disease, can be prevented from crawling up the sides by installing copper tape on the legs and other deterrents.

Costs can vary from as little as $25 to a few hundred dollars, depending on the size of the setup. The simplest design, which would produce six heads of lettuce a week, only requires a 5-gallon plastic container, six 2-inch net pots and hydroponic fertilizer.

To produce 24 heads over a monthlong period, gardeners need to stagger the planting of seedlings (in different containers of the the same size) for four consecutive weeks. A larger system, entailing a 4-by-4 wooden platform built of plywood, can yield 36 heads per week and costs about $100 in materials. Use a heavy plastic liner to contain the water within the wood frame; cover it with another piece of wood, drilled with holes from which to suspend the net pots.

Since it’s difficult to eat or give away 36 heads of lettuce per week, Uyeda suggests people mix and match the type of vegetables they grow instead of planting just one kind of crop — for instance, green onions, lettuce and arugula in each set-up.

In a demonstration, Uyeda set up a 5-gallon heavy plastic container with a cover that was drilled with six 2-inch holes. He filled it up with water up to about one-fourth of an inch from the top. He mixed in three fertilizers: 2-1/2 teaspoons of ­calcium nitrate and 5 teaspoons of a hydroponics fertilizer, lettuce blend type (he has been using the Hydro­Garden brand, which recently went out of business). Both types of fertilizer are available online. Then he added 2-1/2 teaspoons of magnesium sulfate (which is simply Epsom salt).

After putting the cover back on the container, he inserted a net pot in each hole so that the bottom 1/4 to 1/2 inch of the pot was immersed in water. Each net pot was filled with a starter seedling planted in potting soil.

After less than a month, the water should decrease about halfway down the container — do not refill it. Roots should be thickly grown and white and descend into the water; if the roots are brown or yellow, replace the water and fertilizer.

Gardeners can use the system on their patios as long as the section gets 4 to 6 hours of sunlight, or they can set it up indoors under a grow light or fluorescent light. Uyeda recommended erecting a shade cloth cover over the system in areas of stark sunlight.

Kratky also advised installing a cover in areas of frequent rainfall to prevent the water level in the container from drowning the seedlings; the best place outdoors would be under the eaves of the roof, a patio or garage that gets enough sunlight.

Kratky said he’s somewhat embarrassed to have such a popular method of hydroponics named after him because he didn’t exactly originate it but simplified and improved systems developed in the 1860s by Julius von Sachs, in the 1930s by William Frederick Gericke and others that followed. (He cites experiments that actually go further back to 1699 by John Woodward, in addition to so many others through history.)

He credits his colleague professor Hideo Imai, who was his boss in 1986 on a research project, for sparking his own experiments in hydroponics.

Kratky’s use of widely available 2-inch net pots ­distinguished his system from that of his predecessors, because the pots simplified everything, he said. He patented his systems in 1995 and 1996, though the patents have expired and anyone can use his methods freely, he said.

He’s enthused that people are continually coming up with better ideas to improve hydroponics, and making it cheaper and more efficient.

It was MHPGardener on YouTube who used the system and dubbed it the “Kratky method,” bringing greater recognition to Kratky. In 2012, MHPGardener produced five instructional videos demonstrating the method; they were viewed by over 2 million users, Kratky said.

He cited his flagship article, “A Suspended Net-Pot, Non-Circulating Hydroponic Method for Commercial Production of Leafy, Romaine, and Semi-Head Lettuce,” published in 2010 by CTAHR that can be used for instruction today. Go to 808ne.ws/kratkymethod for a PDF.

Is there a gardening topic you’d like to read about in the Garden Variety column? Email Pat Gee at pgee@staradvertiser.com with your request.