It happens to every grower at some point, that tray of microgreens that just doesn’t germinate evenly. Why does this happen? There are a number of possible causes of spotty trays. In order to fix the problem you’ll need to diagnose it first.
6 Reasons Why Microgreens Fail
Low germination rate from seeds
Mold growth/damping off
For larger seeds, improper soak time
Bad seed lot from the seed supplier
Uneven Seeding of Microgreens Trays
This problem usually results from one of two reasons.
Uneven planting surface
The first step is to ensure that your planting surface is even and well tamped down. Fill your tray with the desired amount of growing media. Smooth it as evenly as possible with your hand. At the same time, you are using another flat bottom tray or cut board to tamp the media firmly BEFORE you water. I like to use a mesh tray for tamping down. It presses the surface evenly without compressing it too much.
Improper seeding rate
Use the proper seeding rate for your tray size. Weigh or measure your seeds before you spread them out. Check out the Ultimate Microgreen Growing Resourcefor suggested seeding rates. To save a little time, you can weigh out a type of seed and then get the volume needed, so you don’t have to weigh every time.
**For example, 30 grams of my radish seed mix is just about three tablespoons. Every time I get a new batch of seeds, I weigh to get an accurate volume.
Low Germination Rate of Seeds
Low germination rate from seeds can be a problem that takes a little work to diagnose.
Buy high-quality seeds for optimal germination.
When purchasing seeds, check the germination rate in the seed description. Growing microgreens from any seed lot with less than a 75% germination rate is not recommended.
Older seeds can have a lower germination rate than new seeds.
To run a germination test on your seeds, place 10 to 20 seeds in a wet paper towel and let them sprout to check their vitality. If only 14 out of the 20 seeds germinate, your germination rate is 70% and too old to be used for microgreens.
Mold growth, damping off and improper soak time are all related problems.
The two leading causes of mold growth and damping off are lack of airflow and “dirty” seeds.
Ensure your growing space has adequate airflow by installing fans in your grow room. Humidity can also be an issue, so, depending on where you live, using an air conditioner or dehumidifier can work wonders. I like to keep my grow room between 65 and 80 degrees Fahrenheit. A room kept cooler than that can cause the plants to grow too slowly. Higher than that, the chances of mold increase dramatically.
Some crops are more prone to mold than others. Larger seeds are the usual culprits. They are more likely to be “dirty” because they have a greater surface area to hold mold spores. They are also the seeds that we soak before planting.
This resource here is our growing guide, which also shows recommended soak times. Always use the lowest soak time to start as you figure out what works best for you. Sterilizing some crops is the best way to prevent mold and damping off.
If stacking, thoroughly water those trays with a gentle mist and DO NOT TOUCH them for at least a few days. No peeking under the trays! That seed is sticky from being wet and will stick to those trays until it has some growth anchoring it to the soil. The mist will keep the seeds from being moved around by the water.
Proper bottom watering, if all of your seeds are not staying saturated, you may have sporadic germination. Make sure your surface is level, and you are adding enough water to reach all corners of the tray.