- Author: Michael Cahn
- Author: Richard Smith
Once again, we are experiencing a prolonged heat wave in the Salinas Valley. Maximum air temperature in the King City area reached 112 °F earlier in the week (Fig. 1). Recent maximum air temperatures in South Salinas have been far greater than the average temperatures recorded for the same period during previous years (2015 through 2019).
Although this heat wave will probably wane in the next several days, the central coast region will likely experience periods of record setting temperatures in the future. There are several concerns about how prolonged elevated temperatures affect cool season vegetables. Heat can cause immediate damage to plant tissue when temperatures of the plant surfaces become too high and cause cells to die (Fig. 2). In addition, sustained high temperatures can affect plant growth and development. For instance, in lettuce damage can vary from obvious burning on the edges of leaves from too much heat load (Fig. 3) to more physiological issues that result in poor head formation in iceberg (e.g. puffy heads). In broccoli, if heat damage occurs when heads are forming, it can result in uneven bead sizes when the head matures (Fig. 4). Excessive heat can result in wilting in cauliflower (Fig. 5) during high temperatures and expose curds to sunburning or cause discoloring (Fig. 6.) In the past two years, we have observed that excessive heat can stress lettuce plants and make them more susceptible to infection with Pythium Wilt (Pythium uncinulatum). That was particularly evident in the 2020 heat spells. If there is inoculum Pythium Wilt in the soil, stress caused by heat on the plant can set off infection (Fig. 7).
A previous article presented strategies for maximizing evapotranspiration rates to keep crops cool. Evapotranspiration (ET) is the process in which liquid water vaporizes from plant leaves and moist soil surfaces and is lost to the surrounding air. As liquid water vaporizes, heat is also lost from the surfaces of leaves and soil and from the surrounding air, which cools the crop. Assuring that crops have adequate soil moisture during the hottest period of the day (generally 11 am to 4 pm) can keep plants as cool as possible. Insufficient moisture to meet crop water requirements can result in stomates of the leaves closing and decrease transpiration rates. Limiting transpiration would raise leaf temperatures, potentially to temperatures greater than the surrounding air.
Hence, a good strategy to prevent heat damage to vegetable crops is to water fields that have not been recently irrigated. Also, keep in mind that during the last few days daily reference ET increased substantially due to the high air temperatures and so more water is needed than normal to replace the amount of moisture that crops transpiration. In South Salinas, for example, the CIMIS station showed that daily reference ET increased from 0.18 inches per day in late August to 0.25 inches per day during the heat wave, approximately a 40% increase in water demand (Fig. 8).
Irrigations do not need to be very long, as much as they should supply the crop with enough water to refill the soil profile to the depth of the root zone. Irrigating more frequently for less time would be a better strategy than irrigating less frequently for more time, since the soil has a limited capacity to store water in the root zone. Over-saturating the soil during high soil temperatures through heavy irrigations could worsen infections from soil-borne pathogens.
The CropManage online decision support tool can assist with determining the amount of water to apply and frequency to irrigate for most vegetable crops produced in the Salinas Valley. The software allows one to customize the recommendations for the development stage of the crop, soil type, and irrigation system characteristics.
Finally, for crops irrigated by sprinklers, short irrigations during the hottest time of the day can reduce air temperatures. This might be a good strategy for vegetables that are in a stage of development that is very susceptible for heat damage, such as cauliflower close to harvest.
- Author: Michelle Leinfelder-Miles
- Author: Rachael Freeman Long
Last week, I visited a baby lima field in the southwest part of San Joaquin County that had overall poor pod set. Pods were filling lower in the canopy, but flowers had not set higher on the plants. The field, which was planted in late-June/early-July had an excellent stand, and ostensibly, good fertility and moisture status (Figure 1). There were two possible reasons for the poor pod set that immediately came to mind: 1) lygus damage and 2) heat stress.
I checked data from the CIMIS stations nearest to this field, which are the Brentwood and Manteca stations. Between May 1st and August 31st, the Brentwood station recorded 16 days with a temperature over 100°F, and the Manteca station recorded 9 days over 100°F. Most notably, the heatwave in mid-August struck at perfectly wrong timing. This field was about 50 days after planting, which is generally the prime time for bean flowering. The heatwave brought daytime temperatures over 105°F and nighttime temperatures that barely, if at all, dropped below 70°F. In fact, it is the high nighttime temperatures that will impair pod development by hindering pollen movement and rendering it sterile. This is not just the case for limas; it can happen with other dry beans, as shown by Rachael Long in this blog post from a couple weeks ago. Had the heatwave occurred earlier in the summer, it could have caused a split set, which is not desirable. The mid-August timing, however, means that day length is now too short for further pod development, and yield will likely be lower than expected.
Naturally, one should ask what would be considered a ‘high' nighttime temperature? Rachael remembers having a conversation roughly 20 years ago with UC dry bean breeder of that time, Steve Temple, who said that nighttime temperatures above 68°F will cause poor pod set. This is corroborated by recent work out of the University of Delaware that indicates nighttime temperatures of roughly 70°F impairing pod set. CIMIS recorded nighttime temperatures in that range. The grower indicated that some nights stayed closer to 80°F, which is the temperature at which breeders screen varieties for heat tolerance.
So, what can a grower do during a heatwave? Obviously, we can't control the weather, but it's important to ensure that beans are not moisture-stressed at bloom, and especially not when bloom occurs during a heatwave. Check the top 12 to 24 inches of the soil profile, and irrigate if the soil is dry. If in doubt about how much water is needed, check the reference evapotranspiration (ETo) and irrigate to replace at least 120 percent of your daily ETo. Daily August ETo in the San Joaquin Valley ranges from 0.2 to 0.3 inches per day, so growers would want to apply 120 percent of that amount. In the field that I visited, the crop looked very healthy and non-stressed, so there is no clear suggestion for a management practice that could have saved the set during our recent heatwave.
For more information on lima bean production in California, please see the UC production manual.
- Author: Michelle Leinfelder-Miles
- Author: Rachael Freeman Long
Last week, I visited a baby lima field in the southwest part of San Joaquin County that had overall poor pod set. Pods were filling lower in the canopy, but flowers had not set higher on the plants. The field, which was planted in late-June/early-July had an excellent stand, and ostensibly, good fertility and moisture status (Figure 1). There were two possible reasons for the poor pod set that immediately came to mind: 1) lygus damage and 2) heat stress.
I checked data from the CIMIS stations nearest to this field, which are the Brentwood and Manteca stations. Between May 1st and August 31st, the Brentwood station recorded 16 days with a temperature over 100°F, and the Manteca station recorded 9 days over 100°F. Most notably, the heatwave in mid-August struck at perfectly wrong timing. This field was about 50 days after planting, which is generally the prime time for bean flowering. The heatwave brought daytime temperatures over 105°F and nighttime temperatures that barely, if at all, dropped below 70°F. In fact, it is the high nighttime temperatures that will impair pod development by hindering pollen movement and rendering it sterile. This is not just the case for limas; it can happen with other dry beans, as shown by Rachael Long in this blog post from a couple weeks ago. Had the heatwave occurred earlier in the summer, it could have caused a split set, which is not desirable. The mid-August timing, however, means that day length is now too short for further pod development, and yield will likely be lower than expected.
Naturally, one should ask what would be considered a ‘high' nighttime temperature? Rachael remembers having a conversation roughly 20 years ago with UC dry bean breeder of that time, Steve Temple, who said that nighttime temperatures above 68°F will cause poor pod set. This is corroborated by recent work out of the University of Delaware that indicates nighttime temperatures of roughly 70°F impairing pod set. CIMIS recorded nighttime temperatures in that range. The grower indicated that some nights stayed closer to 80°F, which is the temperature at which breeders screen varieties for heat tolerance.
So, what can a grower do during a heatwave? Obviously, we can't control the weather, but it's important to ensure that beans are not moisture-stressed at bloom, and especially not when bloom occurs during a heatwave. Check the top 12 to 24 inches of the soil profile, and irrigate if the soil is dry. If in doubt about how much water is needed, check the reference evapotranspiration (ETo) and irrigate to replace at least 120 percent of your daily ETo. Daily August ETo in the San Joaquin Valley ranges from 0.2 to 0.3 inches per day, so growers would want to apply 120 percent of that amount. In the field that I visited, the crop looked very healthy and non-stressed, so there is no clear suggestion for a management practice that could have saved the set during our recent heatwave.
For more information on lima bean production in California, please see the UC production manual.
- Author: Ben Faber
Reflections on Water – People and Trees
Coastal California is a hard environment to decide when to irrigate. Fog and rain vary from season to season and day to day. Depending on the proximity to the coast and elevation, average rainfall in Ventura is about 18 inches. That is the average of some years when we get over 40 inches with those when we get 4 inches. Below average is more the norm than above that figure. Late rains into June can happen, but the latest significant rains can also happen in January. So what is average? And based on rainfall, how do you know when to irrigate?
May grey/June gloom adds to the confusion of what might be an appropriate irrigation cycle. That cool, moist, low wind condition fog reduces water use by plants. Fog drip also adds soil moisture that the plant can use. But, as soon as the fog lifts, the wind kicks in and sucks out the soil moisture.
Water moves from the soil, though the roots, up through the plant stem and through the leaves. It's pulled by the conditions outside the leaf. The longer the air outside the leaf is dry, windy and warm, the more water is pulled out of the plant. And then the plant pulls it out of the soil to replace the water lost from the plant. It's called the cohesion-tension theory of water movement. Water molecules stick together and pull themselves along, the way a train locomotive pulls a string of freight cars. This happens whenever the conditions outside of the leaf are “drier” than inside the leaf. It happens in the winter and summer, when the soils are cold and when they are warm. It's a passive, physical process.
When plants lose water through their leaves, it's called transpiration. It's mediated by stomata in the leaves. These openings or pores are similar to the pores in our skin. People lose water off their skin and it's called evaporation or sweating. Water loss from leaves is similar to water loss from skin.
Evaporation from the skin and from leaves cools the surface. This cooling helps prevent heat stress. The leaf and skin both act as radiators. When this water loss stops, both plants and humans can go into heat stress. So water loss has an important function in both plants and humans. For plants, the stomata also need to be open in order to take in carbon dioxide to make sugar by way of photosynthesis.
The weather factors that drive water loss – water that needs to be replaced or the bodies begin to into heat stress – are the mount of light (day length, cloud cover), relative humidity (it dries faster when air is dry and it's slower when it's humid – think desert versus Florida), and windy (more wind, more drying). When water can't be delivered fast enough to the leaf, it wilts, when the human body starts drying out, the skin wrinkles and dries out. In both cases, water needs to be taken in to reverse the loss.
Temperature is important in water loss, but not as important or as much as the other drivers or humidity, day length and wind. When it's cold, leaves and skin both dry out – think freeze-drying, a very successful process for removing water from fresh food to make a light, backpacking food. Often humans respond more to temperature than these other driving factors of water loss. If it's cool, it's not necessary to irrigate the trees. A common grower refrain is, “it's winter, I don't need to irrigate.” After five years of drought, we know better about winter irrigation.
However, this “winter and it's cool, so it's not necessary to drink water while working outside” refrain is common, too. And this can be a real human health problem. Dehydration is something serious and we should all be aware of the need to drink water during these cool, windy days of spring.
Heat stress and irrigation are both more complicated than just being aware of the weather, but below are some helpful guidelines from Cal/OSHA to follow to avoid heat stress in humans. Hey, also might not be too far off for plants, as well.
- Author: Ben Faber
Plants lose water through their leaves and it's called transpiration. People lose water off their skin and it's called evaporation or sweating. When a plant stops losing water and when people cant produce enough sweat to cool off, both overheat. The weather influences that drive this water loss - water that needs to be replaced or the bodies begin to go into heat stress - are the amount of light (day length, cloud cover), relative humidity (it dries faster when air is dry and slower when humid - think desert versus Florida), and windy (more wind, more drying). Temperature is important, but not as much as these other drivers. Think freeze-drying - a very successful process for removing water from food. Often humans respond more to temperature than these other factors and figure, when it's cool. it's not necessary to water their plants, themselves or their workers.
Heat stress is more complicated than this, of course, but below are some helpful guidelines to follow to avoid heat stress:
https://www.dir.ca.gov/dosh/heatillnessinfo.html
Cal/OSHA HEAT ADVISORY
When employees work in hot conditions, employers must take special precautions in order to prevent heat illness. Heat illness can progress to heat stroke and be fatal, especially when emergency treatment is delayed. An effective approach to heat illness is vital to protecting the lives of California workers.
California law requires employers to identify and evaluate workplace hazards and take the steps necessary to address them. The risk of heat illness can be significantly reduced by consistently following just a few simple steps. Employers of outdoor workers at temporary work locations must be particularly alert and also plan for providing first aid and emergency medical services should they become necessary. All workers should be accounted for during and at the end of the work shift. Heat illness results from a combination of factors including environmental temperature and humidity, direct radiant heat from the sun or other sources, air speed, and workload. Personal factors, such as age, weight, level of fitness, medical condition, use of medications and alcohol, and acclimatization effect how well the body deals with excess heat.
Heat Illness Risk Reduction
1. Recognize the Hazard. There is no absolute cut-off below which work in heat is not a risk. With heavy work at high relative humidity or if workers are wearing protective clothing, even work at 70oF can present a risk. In the relative humidity levels often found in hot areas of California (20 to 40 percent) employers need to take some actions to effectively reduce heat illness risk when temperatures approach 80 F. At temperatures above 90 F, especially with heavy work, heat risk reduction needs to be a major concern.
2. Water. There must be an adequate supply of clean, cool, potable water. Employees who are working in the heat need to drink 3-4 glasses of water per hour, including at the start of the shift, in order to replace the water lost to sweat. For an eight-hour day this means employers must provide two or more gallons per person. Thirst is an unreliable indicator of dehydration. Employees often need ongoing encouragement to consume adequate fluids, especially when the workload or process does not encourage breaks.
3. Shade. The direct heat of the sun can add as much as 15 degrees to the heat index. If possible, work should be performed in the shade. If not, employers where possible, should provide a shaded area for breaks and when employees need relief from the sun. Wide brimmed hats can also decrease the impact of direct heat.
- Acclimatization. People need time for their bodies to adjust to working in heat. This “acclimatization” is particularly important for employees returning to work after (1) a prolonged absence, (2) recent illness, or (3) recently moving from a cool to a hot climate. For heavy work under very hot conditions, a period of 4 to 10 days of progressively increasing work time starting with about 2 hours work per day under the working conditions is recommended. For less severe conditions at least the first 2 or 3 days of work in the heat should be limited to 2 to 4 hours. Monitor employees closely for signs and symptoms of heat illness, particularly when they have not been working in heat for the last few days, and when a heat wave occurs.
- Rest Breaks. Rest breaks are important to reduce internal heat load and provide time for cooling. Heat illness occurs due to a combination of environmental and internal heat that cannot be adequately dissipated. Breaks should be taken in cooler, shaded areas. Rest breaks also provide an opportunity to drink water.
- Prompt Medical Attention. Recognizing the symptoms of heat illness and providing an effective response requires promptly acting on early warning signs. Common early symptoms and signs of heat illness include headache, muscle cramps, and unusual fatigue. However, progression to more serious illness can be rapid and can include unusual behavior, nausea/vomiting, weakness, rapid pulse excessive sweating or hot dry skin, seizures, and fainting or loss of consciousness. Any of these symptoms require immediate attention.
Even the initial symptoms may indicate serious heat exposure. If medical personnel are not immediately available on-site, and you suspect severe heat illness, you must call 911.
Regardless of the worker's protests, no employee with any of the symptoms of possible serious heat illness noted above should be sent home or left unattended without medical assessment and authorization.
7. Training. Supervisors and employees must be trained in the risks of heat illness, and the measures to protect themselves and their co-workers. Training should include:
- Why it is important to prevent heat illness
- Procedures for acclimatization
- The need to drink approximately one quart per hour of water to replace fluids.
- The need to take breaks out of the heat
- How to recognize the symptoms of heat illness
- How to contact emergency services, and how to effectively report the work location to 911.
Photo: Heat Stress to avocado leaves.