It’s seed starting time, but sometimes the first pests we have to deal with emerge from the planting medium we use. It’s time to learn what invisible pathogens might be lurking in your potting soil.

Nothing is more frustrating than having your seemingly healthy seedlings wilt or turn yellow without knowing the cause. But potting soil sometimes hosts pests and pathogens that emerge under the right conditions. Here are ways to decipher what might be happening.

Why Are My Seedlings Failing?

Fungus Gnats

One of the most common pests for seedlings is fungus gnats. They love moist potting soil and feed upon organic matter, including the seedlings’ root hairs, which is what causes the most damage. You’ll most likely notice their presence by seeing the tiny, dark flies flying above the soil surface, or teeny-tiny white larvae in the potting mix.

They appear more frequently in organically rich compost because they also, not surprisingly, feed on fungus. Keeping the soil too moist encourages their proliferation.

If you notice fungus gnats around your plants, one way to positively identify that you are dealing with fungus gnats is to use the yellow sticky traps. They readily fly onto them, and you can identify the culprit exactly. If they are present, allow the top of the soil to dry out a little between waterings. Don’t keep it as wet. You might also want to treat it with Bacillus thuringiensis.

Root Aphids

If your seedlings suddenly look weak, stunted, and maybe even yellow, check for root aphids. Root aphids are present in the soil as eggs, but given the right conditions, they go through several processes and feed upon the tender roots of your seedlings. This is why the impact can be immediate; ha ha ha ha ha ha if you don’t recognize it, you can lose the plants within a short time.

If your plants look peaked and you don’t see another obvious reason, use a spoon to lift one of the seedlings and examine the roots. You might see the tiny aphids on whatever root hairs are visible, or they might be present in the soil around it. Either way, if you can confirm the root aphids, the best way to treat them is with a soil drench. This could be insecticidal soap, a Neem-based product, or a pyrethrin drench; the latter works quickly, but you will need to reapply it.

Damping Off

Damping off is undoubtedly the most common pathogen to affect seedlings. One day, your plants look okay, and the next, they tip over at soil level. It’s typically either a soilborne fungus or a water mold. The soil-borne fungus attacks germinated seedlings, usually after they are at least an inch tall. This is when they tip over overnight. The water mold, such as Pythium, tends to affect germination, and your seeds may never emerge.

How to Prevent Seed Damping Off

Both of these are more likely when the soil is kept too wet and there isn’t enough air circulation. This is a good reason not to let them take whatever they want to crowd your plants. Low-light conditions can also predispose them to this infection.

Unfortunately, once a seedling shows evidence of damping off, it cannot be saved. If it is in a container with other seedlings, remove the affected ones and provide better conditions for the remaining plants.

Although we think our potting soil is fresh and free of pests and pathogens, the reality is that they can be present, just waiting for the right conditions. Just keep an eye on your plants and be ready to act if anything is bugging them.