Many people in the personal hydroponic and AeroGarden community refer to these passive hydroponic jars as "Kratky jars". They are loosely based on the Kratky method of passive hydroponics, but technically they aren't. The actual Kratky method is a 'fill and forget it' method – that is, once you set everything up, you shouldn't fill the water reservoir again. This is a key component in the method, while these jars are much too small to actually do this. That said, this vaguely Kratky-esque method is very popular and effective.
In the AeroGarden community, they are generally used to move plants grown in the AeroGarden out into the jars, generally the last stragglers of a crop, to enable you to replant the garden with a new crop, while taking advantage of the AeroGarden's "spillover" light. However, sometimes you want to start plants in addition to the ones in your garden, and so you need to start them direct from seed. Here, I will lay out the process I used to start some herbs I wanted to grow.
Materials
First, you need to gather the materials you will use. First and foremost, of course, is the container. Generally speaking, the larger the better; it means you will need to refill it less often while allowing plenty of room for the roots, and the air roots, which are important to the proper functioning of this method (more on this later.) I went with quart mason jars, as they were the best intersection of size and cost I could justify. The container needs to be light-proof, either on its own, or by covering it with something. Socks are frequently used to cover smaller containers. I chose to paint mine, with a layer of black gesso, then two layers of pretty chalk paint (it took roughly half of an 8 oz jar to paint my dozen mason jars.) I used a strip of narrow painter's tape down the side to make a little gauge window to see the water level.
Next, you need something to suspend and contain the plant. If you are using AeroGarden pods, this is unnecessary, but if you are starting your own, you need a container. In this case, I have 2" hydroponics net cups, which is the appropriate size to fit in regular-mouthed jars with the metal bands on. You can also get 3" net cups, which fit wide-mouth jars.
Another necessity is something to fill the cups with. A wide range of rocks, pebbles, perlite, and other such things work perfectly. In my case, I chose to use expanded clay pebbles, also known as hydroton or leka. In addition, I took an AeroGarden style sponge, and cut it into 3 pieces and nestled them in the center of the pebbles as a 'platform' for the seeds, that would also help keep them damp.
The final part of my setup was 2 small 5 oz clear cups per seedling, to function as a mini-greenhouse for my germinating seeds.
Method
The first thing I did was paint my jars, so that they would be ready once they were needed. I placed the painter's tape, then did two layers of gesso, and two layers of the color paint. This was extremely efficient at blocking light! To the point it's actually hard to see the water in the 'window' after removing the strips of tape.
Then I made my little greenhouses. I set a net cup in one of the clear plastic cups, then filled it with the clay pebbles, and positioned my piece of coir sponge after cutting out a small divot in which to place the seeds. I only filled the cups about halfway, so that I would be able to add more pebbles once the seeds sprouted and provide them a little more support to their lower stems. Once satisfied with this, I added water just high enough to come in contact with the base of the sponge. I used plain, filtered water for this, with no nutrients, to prevent the growth of algae while waiting for the seeds to germinate and grow their initial roots.
Then, I carefully placed 2-3 seeds in the center of each sponge, and used a second clear cup to cover the first, to act as a little greenhouse, increasing humidity and insulating the sensitive seedlings from temperature fluctuations.
It only took a couple of days for the first seeds to sprout! Once the tiny green cotyledons (seed leaves) were visible, I dripped a little diluted AeroGarden nutrient solution directly on the sponges to feed the seedlings without turning the water into a mess of algae.
It took about a week from the initial sprouting, for the little roots to reach down into the water beneath the net cup. Once they reached this point, they were ready to move into their final jar!
I put a half cap of AG nutrients into the jar, and filled it to the top measure line on the jar (800 ml) which was about even with the bottom of the net cup; then I move the net cup over and filled it the rest of the way with the clay pebbles.
Voila! One happy jar of herbs! (After taking the pic, I recovered it with one of the clear cups, which I will keep in place until it has two or so sets of true leaves.)
Important Notes
There are a few important things to keep in mind when doing this.
First, on the container size. Many herbs, particularly the larger ones like basil, develop a considerable mass of roots. They also use a lot of water, and the more roots there are, the less room there is for that water. Even my quart jars are really on the small side for these plants; half gallon jars really would have been a better choice, but I was stretching my meager budget as it was. I will likely upgrade the larger ones to half gallons in the coming months.
The next thing to consider, is how much you fill the jars. As babies, you'll fill to the level of the net cup, because the roots are only barely protruding below it. However, once there is more root structure, you will want to maintain about an inch of air between the net cup and the water level. This is important for this particular method, because it allows for the formation of air roots. Since the water is not aerated, being a passive method, there is insufficient oxygen for the plants in the water directly, so they compensate by making fuzzy air roots, which draw oxygen from the air in the gap. This is critical in true Kratky method, as the water is entirely stagnant; in this hybrid, refilled reservoir method, it is less critical, but still important. The air roots are extremely delicate, and will drown if they get wet, so be very careful when refilling your jar.
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