A late freeze in May is never positive news, let alone after a challenging winter that Nebraskans have endured this year. This morning, the temperature dropped down to 27 degrees Fahrenheit in North Platte, causing considerable damage to vegetable gardens and flowers.
Several factors will impact freeze damage. The actual temperature and duration of the temperature will be the most important. Location of plant material on protected areas, wide open areas, on slopes, in low spots and upland areas will also impact the severity of a freeze. Putting these factors together requires gardeners to observe specific locations where impacted material is located.
Vegetables are one of the first questions that many gardeners will ask about. When temperatures dip down to the mid 20s, many vegetables will not survive temperatures that cold. As the day warms up after a freeze, foliage and plant parts will start to show a water-soaked appearance. This damaged plant material will start to quickly break down and die off.
Tomatoes, peppers, cucumbers and eggplant are severely damaged at 32 degrees Fahrenheit. For those gardeners who planted these crops already, the damaged will become noticeable very quickly or fatal. These severely damaged plants probably will not produce very much over the gardening season. Severely damaged and dead plant material should be removed.
Before replacing these plants, it is important to harden these plants by placing them outside in a protected area and brought in at night for a couple weeks. This process will acclimate tender transplant vegetables to reduce plant shock when they are finally planted in the ground or in a container.
Rhubarb is another crop that gardeners will ask about related to frost damage and toxicity. Freeze damaged leaves and stems will become soft and turn black. Remove these damaged examples with a sharp, sterile knife down to the base of the plant. If stalks appear to be frost damaged, simply remove them to avoid any chance of possible toxicity that can enter stalks from the leaves. Rhubarb leaves do contain oxalic acid, which is moderately toxic. This part of the plant is not eaten anyway, and needs to be discarded.
Early season crops such as radishes, peas, lettuce, cabbage, potatoes, cauliflower, broccoli, and similar crops are going to be more tolerant of a freeze. These crops may have been spared depending on the factors that I have already discussed. Michigan State University Extension has compiled a very helpful set of descriptions of freeze damage on a variety of vegetable crops.
General freeze damage temperature levels for frost tender, moderately frost tender and frost hardy crops are also included. This resource can be downloaded and viewed at
https://www.canr.msu.edu/news/freeze_damage_in_fall_vegetables_identifying_and_preventing for anyone to print and keep for future reference.
Directly seeded warm season crops, including cucumbers, melons, pumpkins, squash and okra should not have been planted yet since the soil has not warmed up to 60 degrees Fahrenheit.
If these crops have been planted, they may not germinate. If these crops have germinated, severe damage of seedling death will not be uncommon. These crops will probably need to be replanted.
For a complete listing of vegetable crops and their minimum soil temperature for directly seeding vegetable crops or planting transplant vegetables, refer to this University of California Extension table at http://sacmg.ucanr.edu/files/164220.pdf to view or download. The optimal growing temperature and maximum growing temperature is also listed for each crop as a quality reference to keep from year to year.
A soil thermometer is one of the best resources for gardeners when deciding plant vegetable crops. This reasonably priced tool can be purchased at hardware stores, garden centers, retail stores or online. For those who do not care to purchase a soil thermometer or need immediate soil temperature data, the UNL Cropwatch website updates soil temperature across Nebraska. This resource can be found at https://cropwatch.unl.edu/cropwatchsoiltemperature to
find the seven day average for a number of locations to help make informed planting decisions based on soil temperature.
As with all decision making, consider all these factors related to freeze damage, damage characteristics by crop, hardening vegetable transplants, and minimum soil temperatures to plant various vegetable crops. Each of these factors are important, and are combined together to help gardeners make informed decisions for best results.
If anyone has any questions about understanding the information on a seed packet, feel free to contact me by sending an email message to firstname.lastname@example.org, by following my Nebraska Gardener blog at https://nebrgardener.wordpress.com/ or by calling my office at (308) 696-6781.