Growing Microgreens is fun & exciting - as it only takes a week or two to have a crop. Here is an overview of the process:
STEP 1 - Prepare Your Soil, Seeds & Tray, and Plant
Seeds - Two things are important about the seeds - how many seeds and do they need soaked. When planting Microgreens the seeds are planted very dense. As a rule of thumb - for all smaller seeds such as Cabbage, Broccoli Microgreen Seeds, plant 1 Tablespoon of seeds per 60 square inches of tray space. For larger seeds such as Sunflower or Pea Microgreens, plant 1/3 of a cup of seeds per 60 square inches of tray space. Here is a link to a Microgreen Seed Density Calculator.
Any seed that is the size of a Radish seed or larger should be soaked for about 8 hours. The exception to this is Pea Seeds which should be soaked 24 hours in a lot of water. Peas Seeds absorb a lot so soak them in more water compared to other types of seeds.
Soil -- When first growing Microgreens, just use regular potting soil. Do not try any organic or specialty soil (some Organic soils are high in certain chemicals & minerals that may hinder the plant growth). Put a minimum of a half inch of damp soil in your planting tray. Make the soil quite damp but not so damp that it puddles when poked with your finger. Larger seeds (Beet size or greater) could benefit from more soil depth though they will grow fine in a half inch of soil.
Tray -- You could grow your Microgreens in a Ramekin dish, a Pie Pan, a Loaf pan, a food take out container or a landscape tray. Professional growers like to plant the Microgreens in a tray that has holes in the bottom as in the later stage of the Microgreens development, they prefer to be bottom watered. To bottom water you put a second tray under the first tray and pour the water between the two trays so that the water is absorbed by the plants through the holes in the bottom of the planting tray.
Plant -- Sprinkle your seeds on top of the soil in the tray. The only seeds that need to be covered with a thin layer of soil are Beet, Chickpea, & Cilantro Microgreen Seeds. For these three, cover with just enough Soil so that the seeds are covered. After sprinkling the seeds over the soil, gently press the seeds into the soil to ensure that each seeds has good contact wtih the soil.
STEP 2 -- Grow in the Dark
Immediately after planting the seeds put the tray in the dark. The dark could be an inverted tray upside on top of your tray (this would also help create humidity), a thick plastic bag (that light doesn't get through), or a dark cupboard. In the dark the germinating seeds will stretch for the light. It is in the dark the the height of the Microgreens get established. Leave them in the dark until they are, at minimum, an inch tall and they have first two leaves are the leaves are partially open.
While in the dark the best way to water the Microgreens is to mist them with water at every 12 hours. Not so damp that water puddles but wet enough that the soil is constantly moist.
The exception - For larger seeds (Peas, Sunflower, Beets, Swiss Chard, Mung Beans, and Cilantro Seeds), beginning when the seeds are first planted, put a weight on top of your tray (2 - 10 lbs depending on size of tray). Leave the weight there until the seeds have sprouted and then take the weight off and continue to grow in the dark until they have reacher their desired height and leaf development. The weight could be a book in a plastic bag, or an inverted tray with a weight it. The amount of weight isn't as important as just having a weight. The weight causes the germinating seeds to push harder as they try to sprouts and helps the seed haul loosen.
STEP 3 -- Grow in the Light & then Harvest
Take the Microgreens out of the dark and put them in the light. They should have a minimum of 8 hours of light, ideally about 16 or 18 hours of light, per day. Eight hours instead of 16 hours just means they will take longer to grow. The light could be artificial light (ceiling lights in your house or office), a grow light, or natural light through a window.
In the Light the breadth (width) of the plants are established. Though they will grow some in height, this is where the plants fill out & develop.
While growing in the light the Microgreens needed water every 12 hours. Again, not so damp that water puddles but wet enough that the soil is constantly moist. Though the Microgreens prefer to be bottom watered now (defined above), they will grow if they are top watered.
Grow them in the light until (at the latest) the very first signs of the second set of leaves start to appear. The exception to this is Beets, Cilantro & Basil which all can be grown into the second leaf stage. Most other Microgreens will begin to taste bitter after the very first sign of the second leaves developing. Peas Microgreens should be harvested when they are 4" tall. Wheatgrass can be harvested when it is 7" tall and is the only Microgreen that is harvested multiple times.
To harvest the Microgreens - cut the stems off just above the soil (the most nutrients are in the stems right above the soil height) with a pair of scissors or a sharp knife. The harvested Microgreens can be stored in an air tight container for at least a week in the refrigerator.