Winter Greenhouse Growing: A Labor of Love

Uncover insightful tips and strategies that turn the labor of winter food cultivation into a rewarding journey. Your next adventure in gardening awaits.

Navigating the Workload

When I bought a box of 206 frozen, unbaked croissants at a big box store, the baker inquired: “Do you know how much work this is going to be?”

“Yes”, I answered, “I’ve done it before,” and the relieved baker smiled as I walked away, croissants in cart.

When I got my first puppy, helpful, single, childless friends tried to point out how much work a new puppy would be. Having already raised three kids through teenage years I thought a puppy would be a breeze by comparison. When I wanted to go out at night, I didn’t have to plan ahead arranging sleepovers or babysitters and I never had to change a puppy diaper. 

But now I’m thinking about the word “work” and wondering about new gardeners, especially, new greenhouse gardeners. Should I warn them of the potential “work” of growing in a winter greenhouse?  Should I ask, like the baker, if they are prepared for all this work?  

The "Work" of Winter Greenhouse Gardening

Compared to croissants (easy) and puppies (more commitment), growing food in your winter greenhouse is something in between. It is going to take planning, sure, and time to raise a winter crop in your hobby greenhouse. Allowing a croissant to rise, teaching a puppy to “heel” or supplying the conditions to get a lettuce seed to harvestable size takes some energy.

A hobby greenhouse grower takes growing food to a whole new level in the winter and luckily, the “work” required to succeed has been trialed, studied and reported on since the 19th century. And now there is a new book published directly aimed at winter food growing. Laid out in a simple, easy-to-read format, J.M Fortier and his co-author and experienced winter grower Catherine Sylvestre offer The Winter Market Gardener with tips and schedules helpful for home greenhouse growers too. 

In essence, The Winter Market Gardener talks about heating a greenhouse, but not too much, selecting the right crops and staging things for maximum results with a specific winter cover. Success is never guaranteed with baking, puppies, or greenhouse growing but with all the great information in this 2023 New Society Publishers release, the “work” of winter gardening is simplified. 

Top Tips for Winter Food Growing in a Greenhouse

• Natural Day Length Matters: Food grows best when natural day length is over 10 hours long. At less than ten hours a day, plant growth is slow or static, rather than fast and active.
• Disease Management:  Disease is greatly reduced when winter crops are spaced wider than summer crops.
• Airflow is Crucial: Constant airflow through use of a fan and opening of vents on sunny winter days keeps plants healthier.
• Harvesting Strategy: Plants harvested one leaf at a time take up less space than plants harvested whole (think of harvesting a parsley leaf versus a whole head of lettuce).
• Cold Tolerance Varies:  Some winter crops are more cold tolerant than others. Spinach is hardier than kale and kale is hardier than arugula.
• Sugar Concentration: Sugar concentration makes leaves sweeter and more resistant to freezing, so winter crops are tastier compared to the same variety grown in summer.
• Efficient Crop Maturity: All suggested dates to maturity are just that, suggested dates. Starting crops in a nursery setting before planting in your greenhouse will give faster harvests and more efficient use of home greenhouse space.

Light Levels Are More Important than Temperature for Plant Growth

As my dad used to say after December 21 every year, “the days are getting longer now, Donna”. For growing food, the length of day is important and greenhouses with less than 10 hours a day of sunlight means growth stops or at least slows down even if temperatures are kept artificially high. Yes, you may get a new leaf or two under low light conditions, but if you plan to eat spinach or kale or any other winter crop, the plants need to be at edible size before the sun levels are lower than 10 hours a day. In most northern gardens, the period between early to mid-October and early to mid-February are no-grow days for spinach even though it thrives with outside temperatures dipping as low as -11 F (-24 C).

Winter Dreaming Brings Success

According to Fortier, December is the perfect time to start thinking about winter gardening. Ordering seed, for instance, of cold-tolerant plants with shorter days to maturity is a perfect way to spend time in December. West Coast Seed lists Jade Spring Pac Choi at 30-50 days to maturity compared to Joi Choi at 55 days. Johnny’s Selected Seeds lists standard Arugula (survives 23 F or -5 C) including the Fortier suggested cultivars Astro and Esmee at 20 days to maturity.

Baby kale is seeded from mid-September indoors for a winter harvest between late October and late April while Evergreen Green Onions are the hardiest bunching onions, ready in 65 days.

A Nursery Area Means More Potential Plant 

Yes, a greenhouse is the perfect place to grow food but a small nursery (either within the greenhouse or under grow lights in your home) will let you raise plants with good roots while allowing fall harvested greenhouse crops like tomatoes to stay in the ground as long as possible before pulling them in the fall. Space Spinach started in a nursery 28 days before greenhouse planting is more uniform and higher yielding according to Fortier and Sylvestre. Starting other crops like kale in a nursery and planting them later in fall or early February also means a quicker winter harvest. 

Getting Started in a Winter Greenhouse

It is almost the new year and gardeners with a new greenhouse are probably visualizing all the great food they will be growing in the next few weeks. Hold your horses. Winter growing is a carefully orchestrated program. It is not as easy as buying young seedlings in January and plunging them into frosty soils without a plan in place.

Winter growing involves knowing what crops are cold hardy (over thirty crops including spinach and kale according to Fortier and Sylvestre.) Winter growing also involves knowing how to use your winter greenhouse to maximum effect (ie don’t grow summer, heat loving crops, like cucumbers in Winter unless you want to spend a fortune on heat and light.) 

And finally, winter greenhouse growing involves visualizing the length of time it will take to grow a winter crop compared to the same crop growing in a summer garden. Staggering or staging crops both in your “nursery” and in your greenhouse, will maximize production in any season, but especially in the winter.