Book Review: Growing Under Cover

Microclimates are areas, large and small, that exist within plant hardiness zones. Within them, temperatures regularly fluctuate above or below the expected norm. They are the likely reason a particular location escapes a frost or why cities, built with concrete and glass, are heat islands compared with the cooling microclimates found in forests. Subtle or sometimes significant deviations in temperature and humidity are found within gardens, a house blocking the north wind or a southern slope that captures the sun's more intense rays. Patios and ponds, walls, a coppice or a clump of trees, all of these can play a role in fashioning a microclimate.

And then there are immense glass greenhouses, sheltering exotic palms and tropicals like those found at botanical gardens, always warm and humid inside; or large-scale high tunnels growing multitudes of hothouse tomatoes that stock grocery store produce departments, no matter that it might be January and snow is piled up in the parking lot. We invent and design ways to adapt and control the environments around us.

In her fourth book, Growing Under Cover, Techniques for a More Productive, Weather-Resistant, Pest-Free Vegetable Garden, Niki Jabbour shares her knowledge and experience for constructing microclimates on a gardener's scale. With the assistance of coverings, be they plastic sheeting, row covers, cloches, cold frames, or mini hoop tunnels, not just commercial growers, but home gardeners can stretch the growing season or grow vegetables throughout the year.

Jabbour is a gardener who has adapted to climate challenges. In Halifax, Nova Scotia an average of 49 days per year are below freezing, and 70 or more include snow. Her one-acre suburban garden sits in the same latitudes as northern Maine and autumn days are short. Due to a natural microclimate, the climate of Halifax is mediated by coastal waters and is warmer than much of the island. Facing the Atlantic Ocean, however, means it is susceptible to hurricanes and high winds. The heat index rarely shoots above 86°F, but with climate change, temperatures will likely increase. Jabbour suggests her many covering techniques might help gardeners adapt to some of these imminent changes in climate.

During all four seasons, in what would normally be considered a short-season garden, Jabbour grows a cornucopia of produce in her 20 raised beds and 14-by-24-foot polytunnel. With the help of a repurposed plastic milk jug, the warmth of late spring arrives early for a heat-loving pepper plant. Rather than waiting until April, she starts spinach in a cold frame beginning in February. On chilly May mornings, tender tomato seedlings stay relatively cozy inside her unheated high tunnel. During the dead of winter, under the cover of straw and topped with an old row cover, she stores vegetables like carrots, turnips, beets, leeks, celeriac, and kohlrabi in a process known as overwintering.

"Mulching root and stem crops for late autumn and winter harvesting is one of the easiest ways to extend the season," she claims. Her garden thrives all year long and, with the aid of various coverings, is protected from weather extremes like hail, as well as from many insects and other critters, including deer, birds, and rabbits.

In her last book, Veggie Garden Remix: 224 New Plants to Shake Up Your Garden and Add Variety, Flavor, and Fun, Jabbour highlighted numerous plants typically grown in much warmer and sunnier climates, but which she regularly cultivates in Halifax. That book won the 2019 American Horticultural Society Book Award. In addition to writing books, Jabbour also hosts a long-running radio show and co-publishes the website, Savvy Gardening. 

Along with manipulating various coverings, the author asks us to recalibrate our traditional conception of the growing calendar. Her framework involves three seasons: warm, cool, and cold. Halifax is in zone 5b. From last frost until first frost is 140 days. This becomes her warm season. The cool seasons, however, include both the crisp nights and mornings of early spring and the encroaching chill of late fall. While her book offers much for the summer gardener, it is at these marked time spans when she most ably finesses nature. With its cooler temperatures and increased rainfall, she extends her warm-season vegetables. Cherry tomatoes cultivated in the open garden in autumn tend to grow smaller; their skins thicken and split, but in a polytunnel, Jabbour reports she continues to grow juicy tender tomatoes for another 6 to 8 weeks. Many thoughtful insights for producing a bountiful harvest, especially inside a polytunnel, are articulated throughout the book.

While her most recent investigation provides a wealth of information about how to garden within controlled microclimates, she also offers guidelines on how to plan and build polytunnels, mini hoops, and cold frames. She explores the pros and cons of different hoop house designs and the effective use of space. Curved sides make it more challenging to cultivate tall plants. The quality of materials, especially relating to plastics, varies tremendously. Thin, construction-grade plastics shred and degrade in a matter of months. Polyethylene, produced specifically in a greenhouse grade, is UV treated and can come with a 4-year guarantee. Polycarbonate, a lightweight substitute for glass, is much tougher than both glass and plastic sheeting and lasts for ten years. Light transmission, heat retention, and installation of materials are important considerations. As for polytunnels, many steps go into building one, and a comprehensive overview maps out the process.

Approaches to constructing a cold frame are detailed, comparing wood, polycarbonate, cinder blocks, and straw bales. An essential feature of a cold frame is the sash. An old window might serve. The angle of the sash in relationship to the sun deserves some thought too. Enough information is provided to get a jump start on research or begin sketching out a plan. The author discusses both kits and strategies for doing it yourself, along with ideas and thoughts about other innovative enclosures like walipinis, bioshelters, and geodesic domes.

Coverings for a slew of purposes are available, both commercially and for the home gardener. Often employed by farmers in their fields, row covers, made with spunbonded translucent polypropylene, shelter plants from cold and frost; help protect crops from critters, including pesky insects; hold mulches in place, and minimize damage from extreme weather such as torrential rains or hail. When summer heats up and temperatures rise, shade cloths prevent cool-season lettuces, such as arugula, from bolting. Insect netting can either be draped directly over plants or wrapped around PVC conduit to make cages. Plastic mulches come in a range of colors, often with a color being crop specific. One example is silver mulch, tested by Penn State, which works to repel aphids, flea beetles, and thrips from peppers and eggplants. Plastic mulches warm soil in spring, reduce weeds, hold moisture, speed ripening, and increase yield. Plastic mulches, however, don't compost like straw or other natural materials.

"The advent of plastic was a game-changer for garden covers. It led to improved materials, larger structures, and season extenders that were affordable and effective. Row covers, PVC, polyethylene, and polycarbonates have made it possible for me to harvest a wide variety of vegetables and herbs year-round from my northern garden."

Indeed, plastics are a man-made, miracle, malleable, lightweight material that, while cheap, don't come free. Here today, but not gone tomorrow. Plastics resist breaking down; consequently, significant amounts of microplastics are found in soil, groundwater, rivers, and oceans.

In Growing Under Cover, Jabbour is generally mindful of the environment and frugal about her plastic use, including methods for extending plastic's temporary life. Cleaning, repairing rips and tears with greenhouse tape, or salvaging old sheets of siding as covers to reuse for other garden purposes are all part of her regimen. She recommends avoiding both thin, flimsy plastics, and what are labeled as "biodegradable films" because when they breakdown they "leave plastic bits in the soil." Nowhere in her book, however, does she mention the environmental impact of plastic once it has been discarded, nor the problems of attempting to recycle it.

There is much to appreciate in Growing Under Cover. It demonstrates techniques that bring the many advantages of greenhouses within the economic reach of most gardeners. Yet, with climate change and other environmental issues facing the planet, this reviewer would have appreciated addressing some of the environmental concerns about the use of plastics.