Seed-starting mix needs to stay moist, but not wet
You can use your finger or a small trowel to check the moisture level of the starting mix. If it feels very wet, don’t water. If it’s dry to the touch, give your containers a thorough watering. Then wait until the surface of the mix feels dry again before watering. How long that takes will depend on factors such as how much light your seeds are getting, and whether you’re using plastic or biodegradable pots (biodegradable pots tend to dry out faster).
If seedlings become too wet or too dry at any time during this process, they may die—so in a sense you’re playing a bit of a waiting game with watering. If you decide not to water and then see that one or more of your seeds has rotted from being too moist, you’ll need to start over by sowing new seeds.
Water every one or two days
As a general rule of thumb, keep an eye on your plants and water every one or two days. How often you need to water will depend on your climate, how warm the room is, and how much light is coming in. To determine if you should water your seedlings, stick your finger into the soil. If it feels dry to the touch, it’s time to water them.
If you are using a spray bottle instead of a watering can to water seedlings, be sure that they get enough water by spraying them until the soil looks very wet. It’s important not to drown the seeds in too much water; this will make them rot or cause mould or mildew to form at the base of the plants. The best way to avoid this is by watering lightly at first and then more heavily once they are established seedlings
Use a spray bottle to dampen your seed-starting mix
We recommend using a spray bottle to dampen your seed-starting mix instead of a watering can, for several reasons. First, the spray bottle is less likely to damage or displace the seeds you just planted. It’s also better for keeping soil moist—not wet—and is an ideal choice for watering seedlings. Finally, it’s usually easier to handle than a watering can.
In general, you should mist your seed-starting mix when it feels dry to the touch and water it thoroughly until water runs out of the drainage holes whenever the top layer of soil dries out completely. (You probably won’t need to water more than once in 24 hours.) In order to maintain high humidity levels around your seeds while they are germinating, cover them with plastic wrap or enclose them in a plastic dome that has been designed specifically for use with seed flats and trays.
Don’t keep the soil soggy
This is where the old adage “less is more” comes into play. Watering your seeds properly might seem easy, but it can be a tricky process once you dig in. While plants need water to grow, too much moisture can actually kill off seeds by encouraging mould and mildew growth on the soil surface (yuck!). So, when watering seeds, make sure that you don’t overwater or keep the soil soggy—just moist enough for germination. Once your seeds begin to germinate and grow roots, try your best to water them regularly so that they can establish a good growing medium in order to thrive.
The most common reason for seedling failure is overwatering. When you first sow your seeds, the soil should be moist but not overly wet. As long as the top of your potting mix remains fairly dry for much of the time, you won’t have to water again until it’s fully dried out. You may even see mould or mildew on the surface of your seed-starting mix if it’s too wet, and that’s definitely a bad sign.
If you do see some surface mould, don’t panic! Just turn on a fan to circulate air around your plants and leave it running 24 hours a day while they’re growing. The mould should disappear within a day or two, as long as it hasn’t had too much time to get thick.
Water germinated seeds more
Once your seedlings are sprouted, you can water them more heavily and become a little less cautious. While you still don’t want to overwater your plants, now that they have more leaves, they’ll be absorbing a bigger volume of water. You should still use a watering can with a long spout, or else the water will splash up from the soil and wet the leaves; those wet leaves can encourage mould growth. Make sure that spout is long enough to reach the bottom of your pot!
Start watering lightly, then increase it
Water your seeds at least once a day. If you’re using a tray, water the tray, not the seeds. This helps ensure that the soil is evenly moist throughout and prevents mould growth. If you’re using pots, water the soil, not the seeds. It can be tempting to pour water directly on top of your seeds—don’t do it! You want to avoid disturbing them as much as possible since this can cause them to come out of dormancy prematurely and die from exposure in their weak state. When watering seedlings, water until you see some moisture come through the drainage holes—this will help make sure that all of your soil is getting evenly watered.