Five Common Mistakes Beginners Make When Growing Seeds (and How to Fix Them)

Growing seeds is a relatively simple process: Bury seeds in dirt, add water, heat, and light from the sun, and in most cases, something is going to sprout. The problem is, most of us conduct this process inside, because we’re trying to get a jump on the growing season. Inside, where there’s no sun, heat is hard to control, and the most your seedlings can stretch out their new little root legs is the size of the plastic cell they’re planted in. As such, it takes some practice to grow strong, resilient seedlings, and beginners make plenty of mistakes. The good news is, most of them are solvable. 

Not enough light

This is one of the easiest problems to diagnose, because it is also one of the most likely problems. Remember, when growing seeds inside, you have to try to replicate the sun. When people estimate how much light they need to reproduce the sun, they almost always underestimate. The tell is easy: Your seedlings will be “leggy” with long stems that aren’t strong enough to support the leaves of the plant. The stem is often curved, almost like the plant has scoliosis. The seedlings may bend towards the light, too. 

The solution is, of course, to add more light. I find too often people starting out seeding for the first time buy one growing lamp to arc over their entire shelf of seedlings, when each tray needs 2000-5000 lumens (that’s a lot of lumens). It’s not just how strong the light is but also how long they’re on: Seedlings need 14-16 hours a day of sunlight. Those lights need to be much closer to your seedlings than you think, too. Six to eight inches above your seedlings is the right distance. 

If your seedlings are already too leggy, you can try a few things. First, if they’re tomatoes or peppers, you’ll be able to bury the stem deep into the soil when you pot up the seedling or plant it outside, and this will solve many problems. If it’s another kind of seedling, like cucumber, they don’t usually require the same strong stems, and will likely correct themself over time. It will help to run a fan in the room and run your hands over the seedlings a few times a day. Both will encourage the stems to strengthen. 

Too much light

You can see how close the lights are to most seedlings, and how much light is applied.
Credit: Amanda Blum

With most plant diagnosis, it can be frustrating to hear, “it may be too much or too little.” This isn’t true with light. It’s really easy to tell if there’s not enough, as above, or too much. Like us, plants can get sunburned. They develop scorch marks with the leaves at the top bearing almost white or golden splotches. The splotches don’t have a pattern to them, either.

Now, seedlings are susceptible to a lot of diseases, fungus, and virus, so you may find yourself concerned with knowing whether this is sunburn or something else. The good news is, it doesn’t matter because the solution is the same: Cut it off. The sunburned leaves can’t absorb light or nutrients and can’t recover, so cut them off entirely, and let the rest of the plant recover. Obviously, move the lights farther away, but don’t overcorrect. While seedlings can get closer to traditional lightbulbs, my experience is they burn easier with LEDs, so while traditional tube lights can be within inches of my seedlings with no issue, I keep LED lights six inches or more away. 

No germination

Of all the seeds you plant, some just aren’t going to germinate. There are a lot of reasons for that, but let’s cover the most likely causes. First, you may have buried the seed too deep or not deep enough. On the back of each seed package are directions for how deep to plant seeds—and it’s not a loose recommendation. While the difference between a seed planted an inch deep or one and a half inches deep might not matter, there’s a huge difference between seeds that need to be lightly covered—like carrots and most flowers—and those that get buried, like tomatoes, peppers, cucumbers, and sunflowers. Another tell is this: The bigger the seed, the deeper you bury it. Zinnia seeds barely get covered, but pea seeds need a depth of an inch or so, and large bean seeds need to be buried deep in the soil. 

This is, unfortunately, one of those problems you need to solve ahead of time. If a seed has been sitting in wet soil for too long, and it hasn’t germinated, it has likely gone rotten. You’d just need to start over; by the time you’ve figured out a seed isn’t germinating, it’s likely only a few weeks since you planted, so you haven’t lost too much time. 

Another reason might be that the seeds are simply expired. Every kind of seed has an expiration date. It can be very short, like alliums (onions), which are good for a year. Experienced gardeners will know that these are merely guidelines; you can always test your seeds by trying to germinate them to see if they’re still OK. 

The seed mat you’re using to heat up your seeds could also be too hot, cooking your seeds. You want the seeds and soil to remain between 70-80°F, but no more. Once your seeds are cooked, they’re goners, so start over with fresh seeds. 

Too much water

You can over-love your plants. Too much water, even if your plant survives, discourages your seedlings from growing strong root systems. Under-watering is always the better solution, so long as the plants have enough to survive, since it causes the roots to reach out in search of more resources, forming stronger root systems. Watering methods for seedlings that allow them to only uptake as much water as they need are the most ideal, such as bottom watering, which leaves a little water in the tray for the soil to wick up. But even if you water from the top, you want soil that is lightly moist, but never soaking wet. 

Yellowing leaves or mushy plants are usually a sign of overwatering. If the plants start to smell bad, mold is likely developing in the water, which is another sign there’s too much water. In those cases, the solution is to go the other way and use less water. Your plants may or may not recover. If you trim away any root rot or squishy leaves and stems, it may recover. 

However, most seedlings will develop some green or white fungus on top of the soil, and that is not necessarily a bad sign. It just happens, and it won’t affect your seedling in any way. 

Not enough water

If seedlings grow with the right amount of light, the stems will be strong enough to support a taller plant, even in a small pot. Strong root systems will develop with the right amount of water.
Credit: Amanda Blum

Droopy seedlings usually need water. Tomatoes, in particular, will start leaning over and will quite literally look sad. This can happen toward the end of the watering cycle, and if not left to fester, it’s not a problem, but it’s a sign that you should water them. You should see plants immediately perk up, within an hour or two of watering. 

If your soil block is really dry, that’s never a good sign. You want your soil to always be lightly moist, not dry. If you’re having trouble keeping your soil moist, you should consider bottom watering, as mentioned above, because it is a consistent method. The soil takes up what it needs.