While it’s certainly possible to produce lots of seedlings without ever dealing with plant lights and heat mats, there are definite advantages to expanding your gardening skills into this area—particularly if you live where late spring frosts or early fall frosts can shorten your growing season, limiting the bloom display and reducing the harvest period for edibles. Giving seeds a jump start with indoor sowing can make the difference between a fantastic show and few or even no flowers at all on some annuals and tender perennials. Even where the growing season is plenty long, starting seeds indoors can reduce your wait for blooms to start popping by several weeks or even more.
There are two major factors that affect your success with indoor seed-sowing: light and temperature. Controlling the humidity can also make a big difference in your chances for success.
Providing the Right Light
For the most part, I don’t advise trying to raise seeds on a windowsill. They will probably sprout just fine if they are warm enough (windowsills can be pretty chilly if your windows aren’t well insulated), but the intensity and duration of the light usually isn’t enough to produce stocky, vigorous seedlings; they will likely get pale and spindly well before you are ready to move them outdoors. So, do consider providing some sort of supplemental lighting.
My very first seed-starting setup was on my kitchen counter, with a stand I made from pieces of PVC pipe. It worked well and was easy to take apart for storage. You can find many neat ideas if you do an online search for a phase like PVC pipe plant light stand. (Here’s a simple one: DIY PVC Grow Light Stand.)
Eventually, I ended up needing more space and finally settled on what I still use today: Origami Foldable Shelves. The one I use is described as having four shelves, but for seed-starting purposes, it’s three levels. (You can see what the whole thing looks like on Amazon*.) The wheels make it easy to move around, the metal finish is easy to clean, and it ingeniously folds flat for storage. Besides using it for seed starting in late winter and spring, I find it valuable for drying seeds and bundles of flowers and herbs through the summer and fall. Mine is about 3 feet wide and 21 inches deep, so it can hold two or three 4-foot light fixtures on each level. (The lights stick out a bit at either end but the seeds don’t mind.) You can find smaller versions too, and they can work well with smaller light fixtures.
There are loads of commercial seed-starting/grow-light setups available, if you prefer to go that route (like these from Gardener’s Supply).
I used to use shop-light fixtures with ordinary fluorescent tubes, but it is challenging to properly dispose of old fluorescent bulbs, so I have switched to using 4-foot LED shop lights, and they work fine. At one point, I decided to try LED grow lights instead, but the two I purchased (these on Amazon*) produce some sort of electrical interference that messes with the monitoring equipment for my solar panels, so I can use them only at night, if I use them at all. Still, these sorts of lights seem to get good reviews from other people, so you may find them worth a try.
I recommend looking for fixtures that hang from S-hooks and chains, because they are easy to adjust. I usually start with them about 3 inches above my seed pots and then raise them as the seedlings grow. With fixtures that have cords and clips instead, you can thread them through the wires on Origami shelves and adjust their height that way. If you are using some other type of shelves, it may be easier to hang the lights and then adjust the height of the pots instead.
I generally turn my lights on around 5 am and off around 9 pm. Anywhere from 12 to 16 hours a day usually works fine. If you don’t want to think about turning them on and off manually, consider getting a timer to do it for you.
Controlling the Temperature
Average room temperature (68° to 72°F) is ok for germinating many indoor-sown seeds. If you keep your house on the cooler side, it’s worth investing in a heat mat meant for germinating seedlings. Most will raise the temperature of the growing medium around 10°F above room temperature, significantly speeding up the germination process for many seeds. I have a couple of 9″ x 20″ mats (like this one on Amazon*), each of which fits well under one standard nursery flat (tray). You can find larger sizes, but I prefer the smaller ones, particularly at the beginning and end of the seed-starting season, so I’m not wasting electricity when I have only a few pots of seedlings. Some higher-end mats come with thermostats, which I guess is a nice added feature, but I’ve never used one of those myself.
As marvelous as heat mats can be, you do need to take care with them. Spend a little extra and buy one that is UL-listed for safety, and test it before you trust it with a whole flat of seed pots. Pick a day when you can be home to keep an eye on it, plug it in where you plan to use it, set a “test” pot (with moist growing medium but no seeds) on the center of the mat, and then place a thermometer into or on top of the growing medium. Give it an hour or two to warm up, then check the temperature every few hours. It should stay around 75° to 82°F if it is working properly. It’s normal for the temperature around the edges to be a bit cooler, but overall, if it’s not at least 5°F above room temperature, or if it is significantly above 80°F, don’t trust that it is working properly. A too-cool mat won’t do nearly as much damage as a too-hot one, but either way, a malfunctioning mat is a waste of energy at best and a safety risk at worst.
While supplemental heat usually speeds up the germination process, it’s not as necessary once seeds have sprouted. In fact, keeping seedlings really warm can encourage soft stems and too-fast growth, and they can have a harder time adjusting to cooler outdoor conditions. Once my seeds sprout, I like to move them off the heat mat. That makes space for newly sown seed pots, and the seedlings off the mat grow more slowly and are sturdier.
Moisture is a key part of the germination process. Seeds need to absorb some water to get things going, but after that, too much moisture can lead to rot and too little can stop the process; either way, seeds that were otherwise viable may die before any growth is evident, or soon after.
Generally speaking, sowing seeds onto already-moistened medium and then watering them in immediately will give them the right start, with no further water worries for a few days, at least. I usually water-in my seeds by dribbling water over them, or sometimes by misting with a spray bottle. Another option is to set the sown pots in a tray with an inch or so of water, let them sit in there for a hour or so, and then let them drain for another hour or so before setting them in their germination spot. Don’t leave the pots in standing water or they will likely stay too wet.
Drying-out during germination tends to be more of a problem, particularly when you are working with very small and/or surface-sown seeds, since they are more exposed to open air than seeds that are covered with growing medium. I like to keep a spray bottle of water handy and mist pots of surface-sown seeds once or twice a day if it looks like they are starting to dry out. Another option is to set those pots in plastic bags (one or two per bag) and close the tops with twist-ties or a press-close seal (as on a Ziploc bag). Or, if you have your seed pots in a flat (tray), you can cover the whole thing with a clear plastic humidity dome. (There are lots of different sizes available, like these on Amazon*.)
Keep a close eye on bagged or dome-covered pots, though, because the high-humidity conditions can encourage the growth of fungi and bacteria—some of which can be harmless and some of which may weaken or kill your seedlings. Leaving the bag open a little, or the dome propped up a little, can help to prevent these problems. It may take some experimentation on your part to find the right balance of humidity and air circulation. Once your seeds start sprouting, remove the bag or dome.
Using a Grow Tent
I resisted the idea of getting a “grow tent” for my seed-starting for years, associating it more with a different type of “grow.” I finally took the plunge last year, hoping it would be a solution to the somewhat dark and cool conditions in the only space I currently have for my seed-starting rack, and it has proven to be an excellent investment. The one I purchased (shown here on Amazon*) is the perfect size to hold my 4-shelf Origami rack with the 4-foot lights. The reflective inside greatly enhanced the light, so I didn’t need to buy additional light fixtures (which I had been considering), and the zippered door and abundance of vents made it easy to control the humidity and temperature inside. It stayed so cozy in there, in fact, that I hardly had to use my heat mats when I had more than one of the light fixtures turned on.
An enclosure like this certainly isn’t required for casual seed-starting, but if you’re sowing many dozens of pots indoors each year, it’s definitely worth considering!
*Just FYI, I do earn a small affiliate fee if you follow these links to Amazon and purchase anything on your visit. Feel free to use your favorite search engine to track down other sources of these items if you’re not ok with that. I don’t get any referral fee from Gardener’s Supply.