Critical Spring Temperatures for Frost Damage
Critical Spring Temperatures for Frost or Freeze Damage
Early spring is the time of the year when apple trees are beginning to wake up from their winter sleep, and is also the time for colder night temperatures that we are sure to see. It's always a good idea to revisit the critical temperatures that can cause frost or freeze damage to fruit trees, specifically apple trees. We are currently at silver tip to green tip here in northern Illinois with a forecast of night temperatures in the low 20's later in the week. Fortunately, our trees haven't progressed to far this spring, but we could see green tip next week. This spring has marked another unprecedented weather pattern that raised our temperatures in early March and is dropping our temperatures the first few days of April again. The early warm temperatures accelerated bud swell, and the lower temperatures that are anticipated present the threat of frost or freeze damage to new green tip leaf tissue if it gets down into the teens. It seems that each spring since about 2012, we have been on the verge of critical temperatures for frost or freeze damage with our fruit trees.
As the trees develop in the spring and buds start to swell, they lose the ability to withstand the cold winter temperatures that they could withstand in dormancy during the cold winter months. The young, actively growing tissue can then be damaged or even killed. Swollen fruit buds can better withstand temperatures in the teens without any damage. As the buds open, temperatures in the low 20s can cause harm, but sometimes leave other buds undamaged. As growth moves from green tip to 1/4” green to 1/2” green to tight cluster to pink in apple trees, temperatures in the upper 20s can cause considerable harm to an early blooming tree. Near bloom, the range between slight and severe damage can be very small. Freezing temperatures of 28 degrees F. for 30 minutes will result in about a 10 percent loss and 24 F for 30 minutes will result in a 90 percent loss, as indicated by the charts down below.
In a radiation freeze with clear, calm conditions, fruit at higher elevations or in the tops of trees will be less damaged than those at lower elevations, since colder air is more dense than warmer air and sinks to ground level, pushing the warmer air up. The percent of flowers killed in a frost may or may not relate directly to lost yield later in the season. With large-fruited fruits such as apples, peaches, plums and pears, the loss of 50 percent of the flower may not be devastating since we may only want a small percentage of the flowers to become fruit, meaning that fruit thinning may be totally unnecessary. So the stage of bud and bloom development determines how susceptible any given fruit crop is when freezes occur. For more information on what those critical temperatures are that can cause freeze damage to trees during development, I have added two charts on the Critical Temperatures For Frost Damage on Fruit Trees from Utah State University below that you can download by clicking on either chart below.
Given the weather patterns we have experienced so far this spring a spring frost could still be possible. Once the fruit has set, then the critical temperatures that can damage the fruit become lower. We will need to constantly assess the stage of development our trees are at over the next weeks and their susceptibility to possible freeze injury.
If we continue in a spell of colder weather, apple trees will continue to develop more slowly, but once they begin showing tight cluster, pink and bloom, the critical temperature rises from the low 20’s to the high 20s, to levels just below freezing at bloom time, which is the most critical time to get frost damage.
Our experience here at Royal Oak Farm Orchard and from scienti fic studies both in Australia and overseas have shown that regular application of seaweed (kelp) digests can increase frost resistance. Low concentration seaweed digest applications at about 10-12 day
periods through out the frost danger period can give 2 to 3 degrees of extra frost tolerance for apples and stone
fruit. A suitably chosen seaweed extract can be the grower’s best defense against late frost at blossom time.
When choosing a seaweed digest several factors need to be considered:
• Seaweed digests are often tailored for use on a particular crop and are fortified with conventional fertilizers or organic sources of nitrogen. Those that have added nitrogen may well reduce rather than increase frost resistance because of the effect of the nitrogen producing sappy growth. They should be avoided when the aim is to increase frost tolerance. Foliar sprays of Fish /Kelp mixtures should be avoided for this reason. It's best to stick with a 0-0-1 Seaweed mix like Neptune's Harvest Seaweed Plant Food..
• Most of the scientific work showing increase frost tolerance by seaweed digest application has been based on alkaline digests but acidic digests also appear to work. It is not clear that fermentation produced extracts work as well. Further, it might be expected fermentation - produced extracts may be counter productive, as the spores present may well be ice nucleators.
• Digests that have a relatively high potassium level may be superior to those with low levels because of the effect of increase potassium uptake by the plant through the foliage.
• For foliar application of a seaweed digest, it needs to be applied as a fine spray. It should have been passed through a ne lter during the manufacturing process.
• The digestion process needs to be carried out carefully as it is desirable that the product once on the buds or foliage is slightly sticky and forms a very thin layer over the leaves.
Seaweed digests increase frost tolerance in several ways.
• They contain a number of plant growth regulators, two of them, (cytokinins and betaines) increasing turgidity of the cell wall.
• They contain sugars, such as mannitol, and also potassium both of which will lower the freezing point of the cell fluid and explain part of the observed effect.
• Extensive work by scientists in the UK seems to indicate that there is something in the digests that triggers a gene responsible for “hardening off” the plants.
Whatever the mechanisms involved numerous studies have shown that liquid seaweeds can reduce frost damage in a wide range of crops. Liquid seaweeds forti fied with potash and/or other minerals are more effective than pure liquid seaweed.
Frost damage can be signi cantly reduced in apple crops by developing a strategy. Liquid seaweed application every 8 to 12 days during the frost danger period should be an important part of that strategy. This would be expected to give an extra 2 to 3 degrees of extra frost tolerance. The first application should be made at least 36 hours before the expected frost. The total number of applications will depend on the location but typically up to two to six applications may be warranted, depending on predicted temperatures.
Scientific Studies Cited:
https://link.springer.com/article/10.1007/s10811-015-0574-9
https://www.frontiersin.org/articles/10.3389/fpls.2019.00655/full
Reference in this blog to any specific commercial product, process, or service, or the use of any trade, firm, or corporation name is for general informational purposes only and does not constitute an endorsement, recommendation, or certification of any kind by Royal Oak Farm, Inc. People using such products assume responsibility for their use in accordance with current label directions of the manufacturer.
Home Orchard Management @ Royal Oak Farm Orchard
Home Orchard Management @ Royal Oak Farm Orchard is a blog for the home fruit tree grower providing information about fruit tree management, fruit tree pruning & training and Integrated Pest Management from the IPM Specialist and Certified Nurseryman at Royal Oak Farm Orchard, a 20,000 tree apple orchard and agri-tourism operation located in Harvard, Illinois.
Click or tap any title to read that post.
