Greenhouse Seeds Guide: How to Start Seeds in Your Greenhouse

Updated: June 25, 2019

Greenhouses are magical places that can turn dried up little seeds into flourishing plants in the blink of an eye. There’s something so special about growing your own plants from seed—each seedling is a tiny miracle, but it’s not as difficult as you might think to coax seedlings out of the soil. I’ve put this guide to seed starting together to help you get started working a little magic of your own.

What you need to start from seed

Seeds are potential. Each of these dried up structures contains two main things: an embryonic plant and enough food for that little plant to feed from until it grows some proper leaves. It’s that simple. Coaxing that young plant out of a seed is where it gets a little more tricky, though. Even the easiest seeds need the right amount of water, light, cover and temperature to stimulate the plant inside to pop out and start growing like mad. Before you buy your first seed packet, make sure you have the following items waiting in the greenhouse:

Seed-starting medium – If you’ve checked out our article on greenhouse growing mediums, you already know about this stuff, but if not, look for a bag of a peat-based seed-starting medium where you buy your seeds and supplies. When you’ve got some seed-starting experience, you may want to experiment with your own lightweight formulas.

Seed trays or containers – Whether you want to use seed trays or containers is going to depend on the type of seeds you’re starting and how many you can stand to lose. When you plant in seed trays, you eventually have to prick out seedlings to repot them into their own spaces. This act can damage their delicate roots, resulting in some loss until you’ve got a good feel for it. I prefer to use small biodegradable pots and sit them inside seed trays to simplify care.

Heat mat – A heat mat isn’t always necessary, but if you’re starting seeds while it’s still cold, consider investing in one. Slow germination doesn’t affect the health of your seeds directly, but a large percentage of your seeds may rot before they germinate if the process takes too long. Put your pots or trays on a heat mat to raise the temperature of the growing medium and speed up germination.

Soil probe thermometer – If you’re using a heat mat, invest in a thermometer with a probe. There are many available in garden stores or you can snatch one out of the kitchen section of your local big box store. Just make sure it will read between 50 and 100 degrees Fahrenheit; this is the range you’ll typically be using.

Plastic domes – It’s not always practical to maintain high humidity throughout your greenhouse, especially if you’re only starting a few trays worth of seeds. Preformed plastic domes can be purchased to go with seed trays, or you can make your own with clear plastic bags. A cheap hygrometer can be placed into the bags to help you keep an eye on the humidity level.

Light – Seedlings often do better with indirect light, but once they’ve emerged, providing some light can help extend short days and accelerate growth. Before you light them, though, check each plant’s lighting needs. Shade or part shade seedlings usually do better without any additional light and may need to be shaded from the midday sun.

Seed sowing 101

Sowing seeds isn’t a tough job, but there are some tricks of the trade that might help you get your seeds growing a little faster. If you’re using individual pots and fresh seed, placing two or three seeds into each pot will yield plenty of usable seedlings. To seed trays, either bore individual holes spaced evenly across the surface or sprinkle seeds gently until you’ve run out of seeds. Very tiny seeds may be easier to distribute if you mix them with fine sand.

Once your seeds are spread, they often need to be covered. You can either poke them into the medium, then cover each hole, or add a layer of medium on top, depending on how large the seeds are. I always poke bigger seeds about a half inch under the surface and cover smaller seeds. Smaller seeds blow around easier; I’ve found covering them helps keep them anchored. Very tiny seeds will usually need to be sown on the top of the medium and left uncovered—be careful when watering these trays or you’ll risk floating small seeds around, allowing them to clump. Always check your seed packet for specific directions since there are a few exceptions to the rules.

When the seeds are sown, go ahead and slip your plastic dome over the well-watered tray or pots and wait very patiently. It can take anywhere from a few days to a few weeks (or six months if you’re seeding things like cactus) to see those first little green leaves emerge from the soil. Check the humidity and soil temperature regularly so you don’t drown or cook your seedlings.

Timing is crucial

Articles about starting annuals or vegetables are usually accompanied by a chart showing you when to start your seeds. These are handy guides once you’re armed with a little local weather information. Get on the web or check with your local university extension to find out what the average last frost date is for your area. You can use zone maps, but the more local the information, the better. Once you know your average last frost date (mine is early May), you can plan for your seedlings.

Depending on what you’re growing, you’ll need to count back four to sixteen weeks from your last frost date to determine the best time to sow your seeds. If you’re off by a week or two, it’s not the end of the world; sowing a little later may save your seedlings from a freak cold snap. Starting seeds too early can be a real problem, unless you intend to keep them in the greenhouse forever—the bigger your plants are at transplant time, the harder it will be for them to adapt to outdoor weather.

Hardening off

When I started experimenting in the greenhouse, nobody told me that I needed to give my plants time to adapt to the great outdoors—though in retrospect it made sense that they couldn’t go from a protected environment like the greenhouse and straight into the garden. But, I’m telling you now. So listen up!

Hardening off is the process of making your delicate little seedlings into big, tough transplants. You can’t do it in a day, so you’re going to have to be patient. Once your seedlings have reached transplant size and all risk of frost has passed, take them out to a shaded, wind-free location (a covered porch with a half wall works nicely). Leave them out most of the day, but bring them back inside before dark. Repeat this for two or three days, then gradually start moving them into brighter and brighter lighting.

By the end of two weeks, you should have your little plants at or near the place where you intend to transplant them. If, on any single day, they show signs of stress such as yellowing, browning or wilting, move them back one step and try again the next day. Throughout the hardening off process, keep your seedlings watered well—this is a big step in their lives and it’s very stressful.

That’s it, those are the basics to starting seeds. It’s not complicated once you’ve tried it a few times and you can make a whole field of plants out of a packet of seeds. Learning to start your own seeds not only makes magic in the greenhouse, it allows you to choose from limitless varieties of plants, many that you’ll never see in the nursery.

Kristi Waterworth
image: –Tico– (Creative Commons BY-NC-ND)
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