You've probably heard that question more than a few times during the dog days of summer. If you're like most growers, your main concern is not for yourself (you can get out of the heat!) but for your crops. Most vegetables, even those we think of as heat-loving vegetables, don't do well when temperatures soar into the 90s and stay there. In this issue of the JSS Advantage, we'll focus on production, harvest, and post-harvest solutions to summer's heat.
Vegetables vary in their sensitivity to heat and in the stage of growth at which heat can be most damaging. For some, the heat-sensitive stage is seed germination; for others, it's flower bud development, fruit set, or some other period. Understanding these stages is important to plant breeders trying to develop heat-tolerant varieties in response to warming summer temperatures. Growers can use the information in scheduling their plantings to avoid the most damaging summer heat.
Germination temperatures are well established. For every crop, there is a minimum, a maximum, and an optimum range for germination. You can find those temperatures as a graph in the Johnny's catalog. You've probably used the minimum temperatures to schedule plantings in spring when the soil is just warming up. Now is the time to keep an eye on the maximum germination temperatures, to be sure your soil is cool enough to get your fall crops up and growing. Soil temperatures can be reduced by irrigating, mulching, and shading the beds for several days before you plant.
Surprisingly, many of the vegetables we think of as cool-weather crops will germinate at extremely high temperatures. Cabbage and cauliflower will germinate at 100F (37.8C), carrots and onions at 95F (35C), turnips at 105F (40.6C). But they won't thrive if the temperature remains that high, because there are other growth stages that are more sensitive to heat.
For example, Cornell University research found that the critical period for heat sensitivity in broccoli lasts only 10 days, during the time when the growing tip of the plant shifts from vegetative growth to flower bud initiation. That occurs about 10 days before the appearance of a tiny crown in the center of the plant, or about three to four weeks after plants are set out. Temperatures above 95F (35C) for more than four days during that critical period will result in uneven, poorly shaped heads. A grower can use this information to figure out the best time to plant fall broccoli. Download report on heat stress and heat tolerance in broccoli.
Varieties also vary in heat tolerance relative to one another. Whereas one variety of lettuce will get bitter when summer arrives, the Summer Crisps keep their sweetness. Some head lettuces do better in heat than others: Green Star, Vulcan, Red Sails, and Panisse. Romaines with better heat tolerance include Jericho and Coastal Star.
Even hot-weather crops vary in their heat tolerance. Whereas some tomato varieties stop flowering in 90F-plus heat, other varieties keep on producing. 'Valley Girl', for one, is both heat and cold tolerant. Johnny's makes it easy to choose heat-tolerant varieties: Just look for the sizzling sun symbol next to variety names.
In hot climates around the world, vegetables and cut flowers are grown under shade cloth to reduce heat and light intensity, resulting in better quality and higher yields. Shade cloth is a weather-resistant woven or knitted fabric that is available in densities ranging from 12% to 90%. The density represents the percentage of light blocked by the cloth; for example, a 47% shade cloth blocks 47% of the light. Most vegetables should be grown under 30 to 50% shade. Shade cloth with density of greater than 50% is generally used for shade-loving plants or as windbreaks.
The key to success with shade cloth is to hang it high enough above the plants, and provide enough ventilation that heat does not build up beneath it. That's easily accomplished in a hoophouse or a specially built shade structure such as those seen at nurseries. An inexpensive alternative is a low tunnel made with Johnny's Quick Hoops Bender™. Shade cloth can be laid over the Quick Hoops™, with the sides uncovered for maximum ventilation.
The shade cast by buildings and tall plants such as corn also can be used to relieve some of the heat for heat-sensitive crops. Choosing varieties with good leaf cover to shade fruits such as tomatoes, peppers, and cucumbers, can be helpful, too.
Providing shade of any type can reduce the ambient temperature around your plants by 10F or more in some cases, enough to keep plants from going dormant in the hottest weather.
Food safety and the heat
Pathogenic bacteria thrive in warm temperatures, so pay particular attention to food safety when harvesting, washing, packing, and storing produce in the summer. The guiding principle in food safety should be to prevent pathogens from getting onto your produce in the first place. A set of standards known as Good Agricultural Practices (GAPs) is designed to do just that.
GAPs help you identify potential sources of contamination such as irrigation water, wash water, personal hygiene, packing shed cleanliness, harvest tools and containers, and manure use. For the most part, GAPs are common-sense rules of sanitation such as washing after using the bathroom and using potable running water to wash vegetables. At this time, GAPs are still completely voluntary. However, some wholesale buyers are requiring growers to undergo a GAP certification process to ensure the rules are being followed. And many people assume that GAPs will eventually become mandatory.
To learn more about GAP and how your farm measures up to food safety guidelines, download the self-audit created by the University of California.
The sooner you can remove field heat from your produce, the better. Produce can be hydro cooled by submerging in cold water before packing it. Water must be clean or you'll do more harm than good by spreading pathogens throughout the harvested produce. Ideally, you also have a cooler where you can store produce until you take it to market. Walk-in coolers can be expensive to buy and maintain, though, and many small growers build their own to save money. Cold temperatures are possible with a Cool-Bot
Keep flowers from wilting
Cut flowers are prone to wilting in summer, especially when they're on display at a farmers market or farm stand on a hot day. And nobody wants to buy a bunch of wilted flowers. To prevent premature wilting, follow good harvest and post-harvest procedures. Here are the basics.
- Above all, clean your buckets and clippers thoroughly. Bacteria that reside in dirty buckets proliferate quickly in summer heat and will clog up the stems and shorten the flowers' vase life.
- Pick flowers early in the morning or late in the evening. Don't pick at mid-day when the sun is beating down on the flowers.
- If it's really hot, fill buckets with cool water a little deeper than normal. The pressure of the deeper solution helps push water up the stem.
- Work quickly, minimizing the time between cutting stems and placing them in water.
- Keep the buckets of harvested flowers in the shade until you can transport them to the packing shed.
- If you are having problems with certain varieties wilting, try using a hydration solution in your picking bucket. A hydration solution is a commercial product that helps the cut stems take up water after they are cut. Two floral preservative companies that sell hydrators are Floralife and Chrysal. Follow the label instructions carefully. NOTE: Certified-organic growers may not be able to use these and other floral preservative products, even though they are used post-harvest. Check with your certifier if in doubt.
- Cool the flowers as soon as possible, either in an air-conditioned room or a 50F (10C) cooler.
- Don't mingle flowers with ethylene-producing fruits and vegetables. And be aware that some summer flowers will suffer chilling injury if placed in a cooler set below 40F (4.4C).
- Before displaying flowers, be sure the water in the bucket is deep enough to keep the flowers hydrated all day.
- Above all, keep the flowers out of direct sunlight at market. Keep in mind that sleeves act like little greenhouses in sunlight, making flowers uncomfortably hot, so you might want to put flowers into sleeves after your customers purchase them.