How to Properly Water Your Indoor Seedlings
Watering your indoor seedlings seems like it should be simple enough. But if you’re new to seed starting, it can start to feel intimidating, with all the advice out there on how to do it correctly. Not to mention the warnings about overwatering and how it will kill your seedlings!
You might get the notion that the soil moisture has to be just right at all times to keep those seedlings alive. Not too wet, not too dry, but the exact consistency of a wrung-out sponge. Otherwise, your seedlings’ roots will rot or they’ll immediately die from damping off disease. Oh, and you also must bottom water every time, or mold will grow on the soil surface and spell certain doom!
I’m sure I’m not the only one who’s done some over-thinking about watering seedlings. Am I watering too much, or not enough? Does that actually feel like a wrung-out sponge? Should I get a moisture meter?
While most of that info you’ll hear about maintaining proper soil moisture for seedlings has truth to it, it’s really not too complicated. Let’s hash out the basics on watering seedlings and go over some practical tips to avoid running into trouble.
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Watering During the Germination Stage
Watering needs are a little different, during the sprouting process. While it’s okay to let the soil dry out a bit on the surface for seedlings with established roots, seeds need constant moisture to sprout.
Many of the seeds you’ll start indoors should be sown around a quarter inch deep or less. If things get way too dry right when they’re sprouting, the seedlings might not emerge. Also, too-dry soil can cause the seed coats to get stuck on the little sprouts. Sometimes, this isn’t a big deal. Your “seed hat” seedlings may just look silly until the true leaves come in. However, if the seed coat is still covering most of the seedling leaves after emergence, it can block the light and cause the sprout to fail.
The best way to keep the soil surface damp during the germination stage is to use a fine-mist spray bottle and give your trays a good mist when the soil starts to look dry. This provides even moisture without washing the seeds around.
If you use a heat mat to help with seed germination, the heat may dry the soil from the bottom up. This isn’t a huge worry until after the seeds begin sprouting. But you don’t want to let it get too dry, since seedlings’ tiny roots like to extend to the bottom of the cells quickly after germination. If you find that the soil looks bone-dry at the bottom or the containers feel extremely light, soak them in a pan of shallow water for a few minutes to rehydrate.
How to Tell When Seedlings Need Water
Most plants, with the exception of succulents, cacti, and specialty houseplants (in which case I am not the person to ask) need consistently damp soil for healthy growth. Soil that’s constantly too wet or allowed to get bone-dry repeatedly can cause stress and disease in seedlings.
So, how do you keep your seedlings’ soil in this Goldilocks zone? Thankfully, it doesn’t need to be an exact science. Plants are pretty forgiving. And no, you don’t need a moisture meter; that would be tedious, if you’ve got your seedlings in tiny plug trays. The key is just allowing that soil to dry some in between watering (but not completely!).
As you gain some experience with raising seedlings, you’ll quickly get more confident with gauging when your soil is dry enough to need water again. But don’t stress over it when you first get started. As long as you’re not letting them completely dry out or watering when the soil is still soaking wet, they’ll be fine.
Here are a couple soil moisture indicators to point you in the right direction:
Soil Color: Seedling soil mixes turn a noticeably lighter color when they’re dry, so soil color is a good way to tell how much moisture is available. Wet soil will be very dark. Adequately moist soil will be somewhere in between, and as it dries more, it’ll become a much lighter tan.
Soil that looks dry on top is a reliable sign it’s time to water, for seedlings in small cell or plug trays. It’s a little trickier for those in larger containers, like 4-inch pots or solo cups. For those, you’ll need to check the weight.
Container Weight: In larger pots or containers, the soil surface may dry out well before the middle from the heat of the grow lights. Often, it will look too dry even when there’s still plenty of moisture at the seedlings’ root zone. Checking the pot weight is the best way to tell how damp the soil is further down.
Essentially, a heavy pot means there’s plenty of moisture, while a lighter pot means the soil is getting drier. It takes some practice to get the hang of testing pot weight for soil dampness. If you’re new to seed starting, it’s a good idea to start your seedlings in small cells or plug trays, then pot them up into larger containers when they’ve grown some. That way, you can practice feeling the pot weight with seedlings that are older and less vulnerable to stress from over or under watering.
How to Avoid Over or Underwatering Seedlings
There’s a lot of talk about overwatering and how this pitfall is one of the most common causes of failed seedlings. While that’s not wrong, a fear of accidentally overwatering can cause seed starting beginners to either underwater their seedlings or not water them deeply enough.
Allowing your seedlings’ soil to get too dry on a regular basis can cause poor growth and stress symptoms like leaf curl and plant edema. If you let very young seedlings wilt from lack of water, they may not recover or may have leaf damage if they do.
Soil pulling away from the edges of the container or soil that is very hydrophobic are both signs you’ve let it go a little too far into the danger zone, with small plug trays or cells. Hydrophobic soil will repel water. That means you’ll see the water bead up and sit on the soil surface for more than a few seconds before it sinks in.
What does overwatering really mean?
To avoid overwatering, you need to understand what actually causes it. Overwatering isn’t going to happen from watering seedlings too much at once, or watering a little sooner than they really need it from time to time. Plants, even tiny ones, are totally adapted to handle wet soil for periods of time. Overwatering problems, like root rot and damping off, happen when you’re constantly watering soil that’s still very saturated.
Do you have to bottom water seedlings?
I’m sure you’ve heard about bottom watering, if you’ve done any amount of research on seedling care. It’s kind of touted as the ONLY acceptable way to water seedlings these days. To bottom water, you place your seedlings in a hole-less seedling tray or shallow pan, then fill it with an inch or so of water. The soil will slowly wick the water from the bottom up, providing deep and even moisture without wetting the seedlings’ leaves or displacing seeds or sprouts.
Some folks swear by bottom watering seedlings and would never do it any other way. Others find it too much trouble to fill trays with water, shuffle seedlings back and forth between trays, and then dump the excess water.
Really, bottom watering is a matter of preference rather than a “best practice.” Top watering works fine, if it’s less hassle for you in your setup. For context, commercial nursery growers don’t bottom water. They walk through the greenhouses with a hose on shower mode and spray all the seedling trays down, and they’re still turning out healthy seedlings.
Personally, I only bottom water during the sprouting phase, when the heat mat dries the soil at the bottom. Once they’re off the heat mats, my seedling cells always seem to dry out at different rates, even in the same tray. I may have some that need water stat and others that are still nice and damp. Top watering makes it easier to water only the seedlings that need it without overwatering others.
If you decide to top water your seedlings, pick a watering vessel that allows you to easily control the flow. A hard stream from a regular watering can disturbs the soil surface quite a bit in small seedling cells, and that may displace your sprouts or expose roots. Plastic condiment bottles with a squirt lid are excellent for watering tiny seedlings. You can usually find these in the grill accessory section or kitchen supply section at the grocery store or other preferred getting place like Walmart/Target.
Does bottom watering prevent mold?
Since mold likes moisture, soil that’s wet at the surface creates a favorable environment for mold spores to grow. There’s a claim that bottom watering helps prevent mold or algae/moss from developing, since you can “theoretically” keep the soil surface dry during the watering process.
In my experience, it doesn’t make a lot of difference. Even if you pull the seedlings out of the water before the soil is wet at the top, it will continue wicking until the moisture eventually reaches the surface. If it doesn’t, you may not be watering for long enough.
Mold and algae on seedling soil sometimes means you’re not letting the soil dry enough between waterings. Often, though, it’s just inevitable with an indoor seed starting setup, and it doesn’t mean you’re doing anything wrong.
Some Best Practices for Watering Seedlings
Here are a few simple tips for keeping your seedlings’ soil in the right range for healthy growth:
–Avoid watering on a fixed schedule.
It’s impossible to come up with an exact timeframe for your watering routine, since the time it takes your seedlings’ soil to dry out depends on factors that are always changing. Things like the temperature and humidity in your growing space affect how often you’ll need to water. The soil may dry faster during cold weather when your heat kicks on more often. During mild, wet weather, it’ll dry a lot slower. That’s why it’s important to go by soil dryness rather than timing when watering your seedlings.
-Aim to water deeply rather than frequently.
Overwatering concerns can cause new gardeners to be stingy with the water in fear of drowning seedlings. But again, overwatering doesn’t happen from watering too much at once. Avoid “spot-watering” your seedlings, or only giving them a dash here and there. That can lead to shallow root development and, ironically, fungal issues from the soil surface being wet too frequently.
Instead, give the soil a thorough drenching each time you water. If you’re bottom-watering, that means letting the seedling containers soak until they’re nice and heavy. When top-watering, pour until you see water running from the drainage holes. This cycle of deep watering and gradual drying is more natural for plants, and it’ll help them develop healthy roots.
-Err on the side of watering too soon, if you won’t be able to check your seedlings for hours.
When you have plans that mean you won’t be able to look in on your seedlings for a while, it won’t hurt to go ahead and water, even if they aren’t quite ready for it. Doing this every now and then won’t hurt the seedlings, and it’s much better than risking them drying out and possibly dying while you’re away. Once it gets to a certain point, seedling mix in small containers can dry out in no time. You don’t want to come home to collapsed seedlings!
-Use appropriate-sized containers.
Pot size is a lot more important than you might think in raising healthy seedlings. Since large pots hold onto water for much longer than small cells, they pose a waterlogging risk for delicate, brand new seedling roots. This is especially true for seedlings that grow and develop slowly, such as peppers.
While starting in small plugs or cells requires some extra chores, like potting up and watering a little more often, it makes maintaining a healthy soil moisture range a lot easier while your seedlings are tiny and fragile.
-Consider warming ice-cold tap water before watering seeds and seedlings.
Cold tap water can be REALLY cold, especially if you’re on a well. Ice-cold water drastically lowers the temperature of your seedlings’ soil, and it may take hours for it to come back to neutral. That’s usually not what you want, since a lot of tender annual veggies and flowers will sprout more quickly with warm soil temperatures. To prevent issues, aim for lukewarm water, or just let it sit at room temperature for a while before using it to water your seed trays.
Frigid tap water might also cause stress for cold-sensitive plants such as peppers, tomatoes, and cucurbits (squash and cucumber family) after seedlings have emerged. While it probably won’t kill them, it can definitely slow their growth. Water temperature is mostly a concern when these seedlings are still little, like the pepper sprouts pictured below.
It’s certainly easy to overthink watering, as a beginner seedling parent. But honestly, it’s a lot harder to mess it up than you’d think, as long as you’re not letting your seedlings get totally dehydrated or habitually drenching them when the soil is still sopping wet. Once you’ve raised a batch or two of seedlings, you won’t be second-guessing about when and how much to water.
