Starting Your Own Seedlings — Part 3: Planting Depth, Watering, Light, and Heat

Starting your own seedlings is both empowering and exciting! We’ve already delved into several options for both the soil and containers you can use, but what are the best ways to water and add light and heat if needed? And what are some tricks for getting the correct planting depth?

Planting Depth and Thinning

The correct planting depth for each seed should be listed on the seed packet. Use a pencil, stick, or the like to make a little hole for the seed to the specified depth. Over the years, we’ve found it helpful to have a reusable planting stick with notches marking common planting depths (e.g. ¼”, ½”). We use the tip of the stick to pull the soil back until the corresponding notch is about at the level of the surrounding soil—not the bunched-up soil we’re pulling back but the level of the soil in the rest of the pot, drop a seed or two in, and cover them with soil to the level of the rest of the pot.

If you plant two seeds and both sprout, be sure to clip off the weaker one once they’re established enough to tell, so the one you keep can grow without competition. This is called “thinning,” and I’m terrible at it. Sometimes we only put one seed in, confident that most will germinate and knowing I might not thin them if both sprout (see thinning post), and replant as necessary. Other times, we plant two seeds per pot, wanting to be sure we have enough seedlings ready by planting time and not wanting to wait for the replanted ones to catch up, and Jen thins them when I’m not looking. 

Watering

Watering seedlings can be tricky. I mean, really, you just put water on them. It’s no big deal and certainly not rocket surgery. However, even though all you’re doing is pouring water onto seedlings, it can be tough to water them without a bunch of soil “blowing out” of each pot—and possibly with it your newly planted seeds.

The first trick is to use a household sprayer to spray water on the top of the soil before seedlings sprout and while they’re small. I don’t know if I’d trust re-using one that had previously been filled with cleaning solutions, but they’re inexpensive enough at any hardware store or online.

The second trick is bottom watering. We’ve moved mostly to bottom watering for two reasons: no soil blow-outs and it’s better for the roots. Bottom watering just means pouring water into the container holding the pots instead of into the pots, letting the soil in the pots soak it up from the bottom. Not only does this avoid the “blowout” scenario, but it also helps make sure the soil is wet all the way down to the bottom of the pot and not just on top.

When seedlings are watered from the top down, the soil in the top of the pot gets priority on water; it’s the first to get wet and often the last to dry out. If not watered deeply, regularly, often only the top portion of the soil in each pot is moist, encouraging the roots to remain shallow since they have no reason to grow deeper looking for water and nutrients and are punished with dry soil for doing so. Plus, it can be difficult when top watering to know when all of the soil in a pot is wetted. Sure one can see when it flows through the pot to the tray, but oftentimes the water has simply found a larger open path through the potting soil and not wetted all of it.

When bottom watering, on the other hand, one knows exactly when all of the soil has been wetted. Since it has to absorb all the way to the top of the soil from the bottom, when the top of the soil is wet, it’s all wet. Also, having the bottom soil in each pot be the first to get water and the last to dry out makes sure the seedlings’ roots grow deep looking for water instead of just staying shallow.

Light

When it comes to providing light for your seedlings, sunlight is the best. If you have a good south-facing attached greenhouse or sunporch, that’s perfect! Seedlings need to be in direct sunlight for most of the day—at least eight hours—or they’ll get long and spindly (“leggy”) in an attempt to grow toward direct sunlight.

If you don’t have such a spot—we don’t—you’ll need to add lights above your plants to keep them from getting leggy and unhealthy. You can buy lights specifically for growing plants, but they’re often expensive. Luckily, you can also use inexpensive fluorescent shop lights. It’s often recommended to use a mix of one “cool” white and one “warm” white bulb in each fixture since it gives your seedlings a closer approximation to differing kinds of sunlight throughout the day. However, using all “cool” white bulbs will work just fine for growing seedlings. The “cool” white bulbs give off the color of light the sun gives during the day and work well for vegetative growth—which is exactly what you want in your seedlings. The “warm” white bulbs give off the color of light the sun gives closer to sunrise and sunset and are better for fruiting and flowering, not something you’re hoping for in your seedlings—but it won’t hurt them either.

Feel free to experiment with different light setups and see if you notice any differences. Just remember to change the bulbs every few years, since their light output diminishes enough to affect plant growth long before the human eye can notice a difference.

Also, ideally, still keep the plants in a window, so they can get all of the sunlight possible, and keep lights quite close to the plants—within a few inches—but not touching, and they’ll do great.

Heat

Only a few commonly grown seedlings need heat to germinate—namely, peppers, tomatoes, and cucurbits—but, for those that do, it’s an essential part of getting consistent germination. Typically, these seeds need temperatures between 70 and 85 degrees to germinate.

Most often, it’s easiest to provide this heat from the bottom up by simply placing your seedling trays full of pots, soil blocks, or plugs on a warm surface. Commercially available heating pads are probably your best bet, and a good soil thermometer inserted in one of the pots lets you know everything is working correctly. Honestly, we’ve used other heating methods and have heard of some inventive workarounds—the tops of a refrigerator or radiators and heating pads meant for sore muscles. We eventually broke down and bought a pad made specifically for starting seedlings. We decided the potential for inconsistent or too much heat from heating pads and radiators that turn off and on and the danger of starting an electrical fire in our house from rigging home health heating pads to run much longer than they were designed wasn’t worth the $80 we’d spend on a proper heating pad that is both safe and gives off consistent heat. 

Each seed packet will typically state the specific ideal germination temperature range for its enclosed seed. In case it just says, “warm soil” or “gentle bottom heat,” here are some specifics: For squash, pumpkins, melons, and cucumbers—the cucurbits—the ideal germination temperature is typically 70-75°, while peppers and tomatoes tend to do best closer to 80°, give or take five degrees. Trying to simplify things for seeds that need heat to germinate, we put all of their pots on the same heating mat that gives soil temperatures between 75 and 80 degrees and have great germination success for all of them.