Tips and Strategies to Maximize Your Winter Greenhouse Growing

An overview of winter greenhouse growing has been covered in our December 2023 blog post, but it seems worthwhile to expand the conversation of winter growing in a home or hobby greenhouse this month as well. For one thing, a greenhouse is not the end of the story. A few other accessories and ideas will make you a more successful grower this winter. 

The nursery, discussed in December, allows you to start spinach 28 days ahead of time under lights and plant it out into the greenhouse in rows six inches (15 cm) apart. Cold temperature hardening before planting in the cold greenhouse helps you successfully grow and harvest super sweet leaves otherwise unavailable to home gardeners.

Accessories and Strategies for Thriving Crops in Cold Temperatures

Most people think the warmer the greenhouse, the faster the crop grows. But as J.M Fortier and Catherine Sylvestre show in their new book, cold crops raised in a well-heated greenhouse compared to cold crops raised in a barely heated greenhouse have about the same yield when you use accessories like row covers in single, double or even triple layers to protect crops over winter.

In a warm greenhouse, with naturally low winter light levels, plants tend to stretch and grow thin and unhealthy. It is better, the authors say, to keep temperatures low and add extra row covers to hold heat near plant crowns at temperatures at or below freezing. Row covers, including brands like Agribon19, are ideal because they are very thin.  Adding up to three layers of row covers will keep spinach and other hardy crops alive even when outdoor temperatures drop drastically. The greenhouse already stops brutal winds that dehydrate otherwise cold-tolerant plants, but the row covers help hold warmth gathered on sunny days. 

Removing row covers on sunny winter days allows more light to reach the plants and also improves air circulation. This reduces the chance of disease on plants growing in winter because dry foliage is less diseased than damp foliage.

Mastering Light and Temperature: The Key to Winter Greenhouse Gardening

There is a problem with the short days in the Northern Hemisphere normally experienced from mid-October to mid-February.  Because plants grow based on the quality and length of light they receive as well as the temperature they experience, there is no point in boosting one environmental condition without boosting the other. Short days with very high temperatures lead to poor growth overall.

Keeping the temperatures cool and choosing hardy crops to grow in winter is the answer. A crop grown in our very short winter days takes more days to mature, so this has to be factored in when choosing the seeding date in late summer or early fall. 

The authors suggest maximizing greenhouse space by starting plants in a separate nursery area before planting them in the cool greenhouse. This lets you leave healthy fall plants like tomatoes in the greenhouse until they stop thriving. Replace these faltering fall crops with well-rooted, nursery-grown hardy plants that are up to 70% of their mature size.

Play around with your greenhouse temperature and keep it just above freezing while raising the most cool tolerant crops like Asian greens, baby kale, baby Swiss chard, mustard and spinach. 

But mostly, avoid trying to grow warm-temperature crops like tomatoes and cucumbers in winter because they will perish unless extra light is added and both soil and air temperatures are high. 

Soil Warming Techniques for Growing Hardy Plants

Even though plants will live, only the hardiest of plants will sprout or grow when the soil is so cold it is partially frozen. This is why heated soil cables come in handy for small hobby greenhouse growers using large planters or soil beds.

It is better to place the heat cables over the soil and under a layer of row cover to hold soil warmth for a few days before transplants are installed from your nursery area. If you have a bright sunny winter greenhouse, the soil can also be warmed or solarized with clear sheets of plastic before seeding radish directly or transplanting nursery crops. Start seeding lettuce, Swiss chard or kale now for planting out as seedlings in February. 

I have, in the past, also placed fabric pots, like Root Pouches, on top of heating cables to warm the soil in the fabric pots. I have had luck growing small potatoes in these fabric pots and harvesting early crops of spuds first thing in the spring before the main crop is ready to plant outside. This isn’t strictly winter growing because it might take until May before the potatoes are ready, but it sure beats waiting until May or June to start growing potatoes outside.

Strategic Planting and Crop Rotation: Maximizing Your Yield

There is never just a single crop that is planted on day 1 and harvested on day 101. Instead, crops are arranged so that they are going in and coming out using more than one date. 

In January, adapting the information in The Winter Market Gardener, specific cool crops going into your hobby greenhouse succeed better if they have been started first in your nursery.  If the soil has been frozen in the greenhouse while you are starting the seedlings in your indoor nursery, use a heat cable, layered on top of the soil before planting and cover the planted crop with row cover over the soil cables to keep the heat in during the first few weeks of establishment. 

By late February, light levels are high enough to sustain real plant growth, instead of just plant survival, and row covers might only be needed on very cold nights. Every grower will have to juggle dates, crops and treatments as the season progresses.

Planning Your Winter Garden 

If you are new to growing or cooking interesting foods, make sure to plant small amounts of everything first until you confirm what you like to eat. After my first trial crop I decided I dislike collards. I find them tough and rubbery, and they are not on my wish list at all. But I love Bok Choy (also called Pak Choi) and spinach, green onions, celery and most kinds of kale. I tolerate mustard greens when they are mixed with lettuce in a salad, and I have a mediocre love for Swiss chard.

Because celery is not as hardy as spinach, I expect it to expire during a super cold January or February spell but by then I have enjoyed it for six months.

I will start planning ahead to schedule winter crops next year and harvest entire plants of Bok Choy or single leaves of cilantro or celery at a time. Once a large Bok Choy plant is removed, smaller green onions and kale plants between them will mature and fill the space suddenly available. The authors of The Winter Market Garden give excellent ideas about crop hardiness and even name specific varieties like Evergreen onions, a green onion they think is the hardiest and most likely to succeed in a winter greenhouse.

Celery in an outdoor garden is usually harvested all at once like a head of lettuce, but in a winter greenhouse simply pick single stems as needed.

Bok Choi is harvested all at once and once removed, it leaves space for smaller plants like onion seedlings or baby kale to be transplanted into the same space.

Like celery, ‘Peppermint' Swiss chard can be harvested one stalk at a time in the winter greenhouse or grown as “baby" Swiss chard and harvested all at once.

Winter Greenhouse Realities: Setting Expectations and Experimenting

All crops will take longer to grow in a winter greenhouse than in a warm sunny summer garden. This new book is not magic or magical thinking. The authors encourage every grower to experiment with their own greenhouse growing conditions and use the information in The Winter Market Gardener as a starting point for this really big adventure: winter greenhouse growing in your hobby greenhouse. 

Remember to bring a blanket outside for yourself as you sit in your greenhouse planning your winter garden. Also, consider ordering “blankets”, i.e. row covers, for your crops because this style of gardening is not your mother’s garden and will not wait until the May 24th weekend. 

Get your planning and ideas flowing now with the help of experts and your own desire to grow what you can in the “off” season.