It hasn’t been an easy growing season for farmers in many parts of the country. The weather has been extreme, and unusual weather events have been widespread. The National Weather Service’s Climate Prediction Center releases long-range predictions of temperature and rainfall for the next two years. These experimental forecasts are not official, although there is an official 90-day forecast released monthly.
For farmers wondering what – and when – the next big weather extreme might be, these forecasts might offer some hope of preparation. The long-range precipitation forecasts show a good probability of higher than average rainfall through autumn for much of the mid-section of the country, with everyone else at just about normal levels.
A few concentrated regions of the nation are predicted to experience significantly less rainfall than normal, although there should be continued excessive amounts of precipitation along the Gulf Coast. But the rest of the country isn’t predicted to veer far from normal precipitation patterns through the end of 2019.
It looks to be an excessively wet spring of 2020 for the Northeast, and a dry one for portions of Washington, Oregon, Idaho and Montana.
Temperatures are expected to be above normal – quite a bit so – for most of the country from September 2019 through 2020. One exception is normal temperatures for August and September 2019 for a chunk of the upper middle portion of the nation, and a lone colder than average period in August in the Dakotas. Looking ahead, 2020 looks excessively hot everywhere.
High temperatures are going to have an effect on crop production. Hot and dry promotes some specific virus, bacteria and fungal diseases, and hot and wet promotes others. Beyond that, higher than normal temperatures cause a slew of other concerns. Extreme hot weather events occurring during the summer can have a significant impact on productivity, as many crops are in vulnerable stages of development.
A summer heat wave just barreled across the country, from Texas to Maine. The dangerous heat brought with it high dew points, which kept the temperatures high all night long, with no relief for people or plants.
When night temperatures don’t cool down, plant cell respiration increases, limiting the amount of sugars and other stored nutrients available for seed and fruit production. In temperatures above the mid-90s, photosynthesis decreases, also with a limiting effect on crop yields.
High humidity reduces water loss in plants and can promote disease development. Excessive moisture levels can affect the taste of fruits and vegetables. Excessive heat can cause plants to bolt more quickly, limiting their harvest window. Above-normal soil temperatures can affect germination of seeds, so crops may not emerge as they typically do. Extreme high temperatures increase water loss and slows growth of crops.
Plant tissue temperatures are normally similar to the air temperatures. Plants keep cool via transpiration, but if there is not enough available water, heat radiation to the cooler air or wind will be their only means of cooling down. During extreme temperatures heat will build up around the plant, limiting this function. Hot, dry winds exacerbate the problem. As plant heat stress increases, so do yield losses. Plant tissues will die when they reach about 115º F.
Leaf scald occurs when temperatures are in the high 90s. Heat lesions are seen on the stems of crops grown on black plastic, as the plastic radiates heat and causes temperatures around the plant to rise significantly.
Tissue death occurs with sunburn necrosis, which occurs when fruit skin temperatures are too high. For cucumbers, this is around 100º, while peppers aren’t affected until fruit skin temperatures reach 105º. Other types of less severe sunburn cause pigmentation changes.
Pollen cannot survive extreme temperatures for long, so reduced pollination will occur even if bees remain active. Misshapen fruits are a sign of spoiled pollen. Blossom drop often occurs with temperatures in the 90s, limiting fruit production, particularly if adequate water is not available. Squash, tomatoes and beans are most prone to blossom drop.
Blossom end rot, caused by a lack of available calcium for developing fruit, is characterized by the lower end of the fruit turning black. Tomatoes, peppers, eggplant, squash and watermelon are all susceptible.
Calcium moves from soil into roots through water. It then travels through the plant via the xylem, not the phloem, and requires transpiration and water movement. If the flow of water is disrupted, the calcium in the plant cannot get to the rapidly expanding fruit cells which require it for normal growth.
If a plant has different sized fruit, all with blossom end rot, then the calcium disruption happened over a period of time. If only fruit of the same age has the disease, then the event occurred only during the crucial stage of development for those fruits. Plant tissue testing may show adequate calcium, but fruits can still be deficient.
Extreme temperatures can cause the fruit to develop more rapidly, outpacing the availability of calcium. If drought stress is also present, the problem is exacerbated, as water is sent to the leaves, not the developing fruit. Over-watering, low soil pH and excessive fertilization are other common causes. High humidity can also interfere with calcium availability. Secondary fungi are often found on the rotted blossom areas.
With excessively wet soils earlier in the growing season, many crops could not develop a robust root system in the compacted, saturated soils. With much of those excessively wet areas of the country now coping with a heat wave, the compromised roots will be struggling to keep up.
Benjamin Phillips of Michigan State University explained in a July vegetable update that a “flash drought” situation occurs due to the underdeveloped roots and the extreme heat. The short side roots of plants will quickly dry out in the heat, and wilting will occur.
Late summer weather can have lasting effects on autumn crop development. Variability in temperature and moisture during this period will take a toll on brassicas, which favor cool temperatures and even moisture levels. Broccoli and cauliflower are most affected by high temperatures during the growing season, and anything above 90º is going to cause issues. Without exposure to cool temperatures, heads won’t form on these crops and they’ll remain in a vegetative stage.
Cauliflower ricing is due to high temperatures, high humidity, direct sun exposure or rapid growth, and makes the head unmarketable. Curd bracts are also a byproduct of high temperatures, as is off-coloring.
Rapid growth due to high temperatures causes head splitting in cabbage. Tipburn, an abiotic disease caused by calcium deficiencies in the inner leaves of the head of cabbage and of Brussels sprouts, results in tissue death. High humidity is one causative factor.
As climate changes, extremes of weather are going to happen with increasing frequency. Crop yields will be affected, perhaps dramatically, and farmers will be coping with concerns they have not previously experienced in their fields. Knowing what the unexpected might bring may be the only way to prepare.