2. Your Seedlings Are Damping-Off
This is usually the problem when stems turn brown and bend at the soil level due to fungi. Make sure to remove the humidity dome right after sprouting; leaving it on creates an environment that pathogens and pests love.
3. They're Infested With Insects
If you have houseplants with pest issues, keep your seedling trays far away from them. The most common indoor seed-starting pest is the fungus gnat. It looks like a small but sluggish fruit fly. Gnat larvae survive only in a moist environment, so cut back on water—once seeds have sprouted, let the top ¼ inch of soil dry out between waterings.
4. You Didn't Plant At The Correct Depth
When planted too deeply and kept too moist and cold, seeds can rot. Plant according to the directions on the packet.
5. Your Seeds Are Too Old
If you’re doing everything right and your seeds still won’t sprout, they may be too old. Seeds germinate best when fresh. Quality seed companies print a “packed for” date on envelopes. Do your research: Some seeds—beans, lettuce, tomatoes, and peppers among them—can be kept stored in a sealed bag in a dry, dark place for several years.