As a departure from my typical recipes, I will be sharing my results of a several month experiment with passive hydroponics. This method is good for growing greens and small herbs in restricted spaces with an almost completely hands-off approach. It requires no power (except any grow lights you use), minimal water (about 1 gallon per plant for the entire lifespan of the plant), and inexpensive supplies (plastic totes, rockwool, nutrient solution).
What are hydroponics?
Hydroponic gardening is soilless; the plant roots are submerged in a nutrient solution dissolved in water for their entire growth cycle. There are several different approaches, of which the most common are:
- deep water culture in which the plants are sitting above a larger pool of water with nutrients dissolved in. A bubbler or aerator circulates the water and keeps oxygen dissolved in the water.
- nutrient film technique, which supplies a thin flow of nutrient solution to the plants. The roots are not completely submerged, and the solution is recirculated continuously using a pump.
- Kratky method, which is essentially deep water culturing without the aerator, removing the need for continuous power. It is this one we’ll discuss.
How does the Kratky Method work?
Plant roots need air as well as nutrients and water. In the deep water culture method, the roots are fully submerged in the nutrient solution, so the aerator’s job is to aerate the water so the plat roots can receive oxygen. Bernard Kratky at the University of Hawaii sought an improvement to this method that didn’t require a powered aerator. Instead of pumping air into the water, his idea is to suspend the plants just above the surface of the water, allowing for some headspace between the entire rootball and the nutrient solution (about 1 inch at first). As the plant grows, the roots grow down into the nutrient solution, drinking it up. The water level decreases, maintaining enough headspace at all times for the plant’s roots to receive both the oxygen they need and the food and water in the solution.
The plants are given an entire lifetime supply of the nutrients and water they’ll need until harvest, removing the need for constant monitoring and re-feeding.
What grows well with the Kratky method?
In general, smaller leafy plants do well with this method, such as
I’ve seen some reports of this method being used to grow larger plants that fruit, but in general, the following are not recommended for this method:
- any root vegetable (Carrot growth would be restricted by the net pots; potatoes and other tubers would be too susceptible to rot.)
- heavy feeding fruiting plants such as tomatoes or cucumbers (They would need a very large container and frequent re-feedings, plus far more light. These are just best grown outside or in a good greenhouse.)
What supplies are needed to grow plants via the Kratky method?
You will need the following:
- Net Pots. Since you’re growing smaller plants with this method, 2″ pots will suffice. I purchased some here.
- Growing Medium. I used rockwool because it’s inexpensive, reusable, and easy to work with. You can also start the seeds in the rockwool and move the entire cube directly to the net pot without worrying about transferring a delicate seedling. Other people use coconut fiber, perlite, or expanding clay pebbles.
- Nutrient Solution. There are as many options and opinions on this as there are gardeners. I was a beginner, so I went with the easiest system to use: General Hydroponics Flora/Bloom series. It isn’t organically derived, so if that’s a concern for you, seek elsewhere.
- Container. The container you choose should reflect how many plants you wish to grow in the container. Plan on about 1 gallon of water per plant for its lifecycle, plus 1 inch headspace. Some people have used mason jars and refilled them when needed, but these are only for growing single plants. I wanted to grow multiple plants, so I found a shallow plastic tote like this with a lid that I could drill holes in for the net pots. I chose a 7 gallon tote, because it held 6 gallons of liquid (for my 6 plants) and still had an inch of headspace left when the lid was placed on the box. The container will also need to either be opaque from the start or made opaque. I bought a clear one and used duct tape to cover the clear plastic — nothing fancy. You need to keep light out of the nutrient solution or algae will grow and compete with your plants for air and nutrients.
- a 2″ hole saw drill bit if you don’t already have one. This is what you’ll use to drill the holes in your tote’s lid for the net pots.
- a grow light if you’ll be growing your plants out of direct sunlight. It doesn’t have to be fancy. I used these , which cost me about $30/each (about $45 now thanks to inflation). They’re not perfect for large plants, but they did the trick fine for my purposes. I also use these lights when I start my outdoor garden seeds each year.
- a place to put the tote(s). I start several hundred plants each year for the garden, so I have a dedicated wire shelving unit for indoor gardening purposes. I suspend 2 4′ grow lights on the underside of each shelf, and daisy-chain them together. I also put them into a power strip that I rigged with a remote to easily turn on and off in the mornings and evenings. Some people put these on timers as well.
- seeds. Your local garden center may carry seeds year-round, but if not, I like Johnny’s Seeds, Rare Seeds, or Seed Savers.
My Experiment and Results
I bought 4 7-gallon shallow totes, planning on 6 plants per box. I covered the clear parts of each one with duct tape (you could also paint it as well, but be careful about the type of paint you get, and paint only the outside). I used a 2″ hole saw to drill 6 holes evenly space (2 rows of 3) into each lid. Put a net pot into each hole to ensure a good fit. The lip of the net pot should be about flush with the lid, and the net pot itself suspended into the box. At this point, the boxes are ready.
Next, the seeds needed to be started. I soaked 24 rockwool cubes in water for about 20 minutes, then put 2-3 seeds in each one. You should stick with the same type of plant in each tote. I chose to try basil, spinach, green leaf lettuce, and cilantro as my tester plants. I chose to cover the seeds with a pinch of seed starting mix in each cube hole, since these seeds like to be shallowly covered to germinate. Some people don’t do this.
Put the rockwool cubes in a tray and place in light for about 12 hours daily. You won’t put these rockwool in the net pots until the seedlings have their first set of true leaves. Soaking the rockwool should give the seeds the water they need to germinate, which can take anywhere from a few days to a couple weeks. In my case, the lettuce was the first to poke up after about 3 days; the cilantro came next at about 7 days; the basil came at day 10, and the spinach completely failed. (I think I’m cursed growing spinach anyway. I’ve tried everything at this point.)
The first set of leaves each seedling gets are called cotelydon leaves–embyronic leaves. They generally look about the same regardless of the plant. These aren’t true leaves yet. the next set of leaves will look like tiny version of the adult plant’s leaves, and will come anywhere from a few days to another week or so after the embryonic leaves. (Gardening requires patience.) You’ll also know the plants are ready to go into the totes when you can start to see some roots poking out the sides or bottom of the rockwool, although I put my basil in the solution before I saw the roots and they did just fine.
When your plants are ready to move to the totes, get the nutrient solution ready before you put them in. If you’re using General Hydroponics’s system, the bottles have the amounts you need to use in a handy table. The amount you need will vary depending on the number of plants you have and the amount of water. I went with the “general vegetative growth” ratio since I wasn’t growing any fruiting plants. It worked great and used an even amount of each of the three bottles (on the order of teaspoons), so this method is extremely cost-effective.
At this point, you can put the rockwool cubes into the net pots in the lid. The rockwool should be just touching the nutrient solution; it’ll soak up what’s needed. You want some headspace to avoid drowning the seedlings.
From this point forward, your gardening is essentially hands off. I just ensured the plants got 12 hours of light daily. It took about 6 weeks before my lettuce was ready to begin harvesting. Growth did seem slow at first, but this is because the growth was mostly in the roots. Once the lettuce started taking off, it really grew fast. I just took leaves off the outsides of the plants to continue growth for a continuous supply of lettuce. Cilantro was a similar story; in fact, it was more successful indoors with the Kratky method than when I grow it outside. The plant was nice and bushy with no bolting; the leaves were a nice dark green with a good strong cilantro flavor. Basil was very slow to do much of anything; a couple months before I got some plants. These plants ended up growing too tall for the shelf they were on and hitting the grow lights. I have pruned them back some, but overall I though their growth and yield was too slow and low to waste space on them with this method in the future. It’s possible that my cheap grow lights just weren’t strong enough to encourage speedy growth.
My grow lights aren’t very expensive or strong compared to the fancier ones that cost hundreds of dollars. This likely was the cause of slower growth in all my plants. The plants themselves did grow nicely and yielded great greens with a fantastic flavor, but the time from seedling to harvest was about 2.5 months, which is a long time.
To make this sustainable for a family of three, I’d have to dedicate an entire shelving unit and have rotations of seedlings and totes going to keep a full supply of greens. I do supplement by sprouting microgreens (alfalfa, radish, mung bean, broccoli), but my family can chew through some salad, so we need a lot of plants.
The nutrient solution is fantastic; I needed very little to get 18 (since all the spinach failed to even germinate) full plants, so I’ll continue to use it. I don’t know why the spinach failed to germinate; I’ll test the seed packet to see if the seeds are just bad. It could just be some ancient and strange curse I have that dooms me to always purchasing spinach.
For anyone wanting to try a beginner hydroponic experiment at a very low cost with little active attention to give, I do recommend this approach.