Join Master Gardeners in the Arlington/Alexandria unit of Virginia Cooperative Extension in a series of monthly articles in 2021 as we explore the topic of climate change and practical actions individuals can take in their home landscapes in response.
By Elaine Mills, Extension Master Gardener
For the past few months, articles in this series have been focused on general gardening practices homeowners can adopt in the face of climate change. This month, we turn to adaptive approaches for home vegetable gardening. These can include the general steps previously discussed for reducing the garden’s carbon footprint, such as minimizing the use of power equipment and purchased fertilizers, building soil health, and conserving water.
At first glance, some changes in our climate appear to be beneficial for growing vegetables. As reported by the Environmental Protection Agency, there has been a large and steady increase in the length of the frost-free period in the United States. Since 1985, the last spring frost has been arriving an average of four days earlier and the first fall frost occurring around three days later. This extended growing season can allow gardeners to diversify their crops and even extend harvesting into the fall and winter months.
Problems of Extreme Heat
Unfortunately, this new longer growing season has its own challenges. For example, the steady accompanying rise in temperatures leads to potential damaging heat stress on plants, affecting food production. Ten percent of summer days are now hotter than a baseline period from 1951 to 1980. At temperatures over 86° F, vegetables will fail to set fruit, bolt prematurely, and suffer from sunscald. The increase in the occurrence of nighttime temperatures over 75° F is especially troublesome, affecting plant respiration, growth, and pollen viability.
Dr. David Wolfe of Cornell University advises that anticipated longer periods of high heat could require a shift to earlier planting dates. Of course, an early start to spring can be accompanied by sudden late frosts, so gardeners should use freeze-protection mulch and be prepared to cover plants with protective freeze blankets. It is also wise to avoid planting on north-facing slopes and low-lying shaded areas that are more subject to frosts. (See the recording of our public education class on “Fall and Winter Vegetable Gardening” for more details.)
Dr. Sara Via, Extension Climate Specialist at the University of Maryland, recommends the selection of heat-tolerant vegetable varieties and the use of white or reflective mulch to help keep soil and plant roots cool. Sensitive crops can be protected with a system of hoops and shade cloth, keeping the ends open for air circulation.
Challenges for Water Management
Vegetable gardeners also face new challenges for water management with increased threats of summer drought, intense rain events, and flooding. The use of sprinklers for watering is ineffective, as a good percentage of water will evaporate before it reaches the ground. The use of drip irrigation and soaker hoses and the practice of deep watering every few days during extended dry periods will deliver this crucial resource directly to the base of plants, encouraging roots to grow deeper.
Heavy spring downpours and summer thunderstorms damage plants, increase the chance of fungal diseases, and wash out or contaminate vegetable plots with various pollutants, making produce unsafe to eat. Waterlogged soil can become compacted as gardeners try to work in it. In areas prone to flooding, the use of raised beds for growing crops is an approach to reduce soil erosion, prevent soil compaction from foot traffic, and promote healthier plant growth.
As discussed in the article on building soil, the use of organic soil amendments can both improve drainage during wet periods and increase the soil’s water-holding capacity during dry spells. Selection of disease-resistant crop varieties and the proper spacing of plants to enhance air circulation and promote drying of leaves can help to prevent the foliar diseases caused by high humidity. (See pages 2-23 to 2-24 of the Pest Management Guide for Home Grounds & Animals for additional controls to prevent or treat diseases in home vegetables.)
New Invasive Threats
Climate change is also bringing heightened pressure from weeds and insects. Increases in severe weather, particularly combined with wind, can facilitate the seed dispersal of invasive weeds. The strong growth response of weeds to elevated concentrations of carbon dioxide, their better overwinter survival, and their early flowering time can give them a competitive edge over desired vegetable crops. The increased concentration of carbon dioxide in the air simultaneously results in a reduction in herbicide efficiency.
Adaptations to increased weed pressure can include a reduction of tilling, which brings banked weed seeds up to the soil surface; weeding when plants are small; and planting cover crops to slow down the invasion of winter weeds. Gardeners can also put down protective layers of leaf mulch, moist newspaper, brown paper bags, or 4 to 5 inches of straw in garden beds. In her own vegetable garden, Dr. Via uses a heavy-duty biodegradable paper weed block that comes in long, wide rolls with soaker hoses placed underneath. She plants young vegetable seedlings through small holes she makes in the paper. (See recommendations of Tried and True Cover Crops which can simultaneously act as green mulch and enrich garden soil.)
Dr. Michael J. Raupp, Professor of Entomology at the University of Maryland, advises that while no one can predict the exact outcome of climate change on insects, it appears that insect pests are also benefiting from warmer temperatures. They are expanding their range northward where winter temperatures are no longer cold enough to kill them and producing more generations per season. Accelerated development may allow juvenile insects to pass through windows of vulnerability and escape death from natural enemies.
This means that gardeners will need to be especially vigilant in their monitoring and to decide on control strategies before pests are detected. It is important to inspect plants as often as possible for egg clusters, larval stages, and adult insects and to handpick or use water sprays to remove as many as possible.
Other control measures include timing plantings to avoid the peak period of insect infestations, using tulle row covers as an insect barrier, and keeping down weeds and grass which can harbor insects and mites. Whenever possible, it is advantageous to avoid the use of pesticides and to encourage beneficial insects, such as predatory wasps, syrphid flies, and ladybugs that can help control the undesirable insects. They can be supported by planting native flowers adjacent to vegetable plots as a source of nectar, pollen, and habitat.
A blog post from April 10, 2019 by Extension Master Gardener Wendy Mills summarized some of the problems faced by leaders of the Organic Vegetable Garden, a demonstration garden of the Arlington/Alexandria unit of Virginia Cooperative Extension, due to unprecedented weather in 2018. The public is welcome to visit this garden, located at Potomac Overlook Regional Park, and speak with Master Gardeners to learn more about the adaptive techniques they are using to respond to the challenges of climate change.
- 2021 Pest Management Guide for Home Ground and Animals, Virginia Cooperative Extension, (See especially pages 2-1 Organic Controls for Insects and 2-23 to 2-24 on Vegetable Diseases.)
- Best Bets Cover Crops
- Christopher, T., ed., New American Landscape. (2011). Portland, OR: Timber Press.
(See Chapter 5, “The Sustainable Edible Garden” by Eric Toensmeier, pp. 103-124.)
- Climate Change Indicators in the United States, U.S. Environmental Protection Agency
- Invasive species and climate change: an agronomic perspective, Ziska, et al., Climate Change (2011) 105:13-42
- Managing a Garden after a Flood, West Virginia Extension Service
- Marinelli, J., ed. (2010). Climate Conscious Gardener. Brooklyn, NY: Brooklyn Botanic Garden.
(See “Climate Footprint of Homegrown Food“ by Amanda Knaul & Susan K. Pell, pp. 72-77.)
- Raised Beds vs. In-Ground Gardens, University of Georgia Extension
- Reed, S, & Stibolt, G. (2018). Climate-Wise Landscaping. Gabriola Island, BC: New Society Publishers. (See Section IX: Food, pp. 245-268.)
- “Regenerative Gardening: Growing food successfully & sustainably in a changing climate,” Webinar #2, Climate & Sustainability Webinars 2020, Dr. Sara Via, Professor and Climate Extension Specialist, University of Maryland