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Greenhouse Gardening 101: A Guide to Starting Seeds in a Greenhouse

Shade cloth in greenhouse

I’m starting a few trays of seeds in the greenhouse this week and thought I’d take some time to write about it. It might seem a little late (it’s May and I’m in Zone 6), but most of the seeds are perennials that I’m planning to put in the ground in the Fall (or whenever the new garden is ready - more on that later). The other reason I’m starting them now is just because I didn’t feel like starting them indoors this year! I thought I’d make it easy on myself and just start them in the greenhouse.

Seeds can be started in the greenhouse both in cool early spring weather or now, after the last frost date. If starting in early spring, you’ll want to watch the forecast for cold temperatures so you can protect the seedlings. Depending on how low the temperature is going to get, you may need to bring them indoors overnight, or cover them, and/or set a heating pad underneath the trays. Some folks also set up a heat lamp near the seeds. Just be sure to use outdoor-rated lights and cords. Starting early will definitely give the plants a jump on the growing season and you should have strong seedlings to plant outdoors once it’s warm.

Starting seeds a little later, after the last frost date, shortens the growing season by a few weeks but also eliminates the work of keeping the plants protected from the cold. In my case, my greenhouse gets significantly more sunlight this time of year than even just a couple of months ago, which makes it a good time for me to start seeds. Since I’m not starting them indoors, they won’t need to go through a hardening off stage.

If you live in a very hot climate, starting seeds in a greenhouse at this time of year may not be an option for you. Your greenhouse may get too hot and unsuitable for seedlings. However, February, March and April are probably ideal months for seed starting in your area.

Greenhouses are ideal environments for seedlings due to the controlled environment, protection from weather and animals, and humidity. Here’s a few tips for successfully growing seeds in a greenhouse.

Select the right potting mix

It’s crucial to use the right potting mix whether starting seeds indoors or out. You’ll want to avoid using garden soil, as it can be heavy and prone to diseases. Instead, choose a high-quality seed-starting mix that is well-draining and nutrient-rich. The mix won’t contain any soil, but is made of light, fluffy materials such as peat moss, vermiculite, perlite, coconut coir, and rice hulls that provide the ideal base for seeds to germinate and grow. Bags of seed starting mix can be found easily at your local nursery or online. To plant your seeds, fill clean seed trays or pots with the seed-starting mix, leaving space at the top for watering.

Protect seedlings from overheating

Tender seedlings can get too much heat or even burn in a greenhouse since greenhouses heat up quickly and and can easily overheat. Take care of your seedlings (and other plants!) by installing shade cloth and running a fan. The shade cloth will filter the sunlight while still providing enough light, and the fan will keep the air circulating. Both the fan and shade will also help regulate temperature, and prevent scorching and heat stress. If you notice young seedlings still looking a little wilted or drying out very quickly, consider adding additional shade that temporarily covers the seeds during the hottest part of the day.

Promote growth using air movement

Seedlings planted out in the garden are naturally pushed around by breezes, and that movement stimulates cell growth and creates stronger stems. A fan in the greenhouse can provide this same movement and benefit to all plants, but especially seedlings. Additionally, the air movement reduces the risk of an overly damp environment and the risk of fungal disease.

Monitor to prevent soil from drying out

After you’ve planted seeds, observe your greenhouse conditions to avoid underwatering and overwatering. You’re looking to provide the ideal growing conditions where the soil is slightly damp, but not waterlogged. When watering, water the soil instead of the seedling, and water gently using a spray bottle or small watering can. Watering early in the day or late in the day helps prevent the soil from drying out too quickly.

For my seeds, once the plants are fairly big and sturdy, I’ll transfer them to felt grow bags for the rest of the season. I’ve found that most plants do exceptionally well in these bags as they’re breathable and don’t dry out quickly. They’re also pretty inexpensive, and for the past few years I’ve been using them as temporary holding spaces for plants until I’m ready to permanently plant them. They’re great for plants I find on clearance, plants I get from friends, and plants that I divide.

In Fall, if I’ve had a chance to prepare my new gardens, I’ll go ahead and plant the young perennials. If the new gardens aren’t ready, I can leave the plants in the grow bags over winter. It’s an easy and flexible way to plan for a future garden.

I just realized that I didn’t say what I’m growing! The two main perennials that I’m planting are Agastache and Veronica Speedwell. They are deer resistant, can withstand dry conditions, and attract pollinators. These are headed to my front yard where I want something that looks nice without a lot of effort. And since we have deer that roam the neighborhood, and since I get busy and forget/don’t have time to water, I need plants that are likely to look good no matter what!

You'll see in the picture that I'm also starting a few morning glory plants. This variety is Ipomoea nil, which is supposed to be non-invasive, unlike the invasive Ipomoea purpurea varieties.



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