Nebraska is well known for being a land of extreme temperature swings. It isn’t uncommon in Fall or Spring to go from a nice day to a blanket of snow in less than two days. When this happens, it can cause damage to trees that were not fully prepared for the change.
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First freezes
In fall, many trees slowly start to prepare themselves for the extreme cold of the winter. This, however, takes time. During 2018 and 2019, temperatures in late October and early November dropped to single digits very quickly, as shown in Figure 1. This sort of quick drop can freeze tissues in the tree before they have had a chance to go dormant. It can also trigger latent infections of pathogenic fungi which may have been kept in check until this point. Damage done in the late fall likely will not show up for some time—even years later if it was done on the main trunk. Evergreen needles damaged in this way will turn brown the following spring.
Late freezes
Similar damage can happen in the spring with a late freeze. The average last freeze in spring occurs in late April for the southeastern part of the state and mid-May for the northwestern part of the state. However, the actual dates of the last spring freeze can vary dramatically each year. In Lincoln, the earliest such freeze occurred March 27 in 1925, while the latest occurred May 29 in 1947.
Interestingly, the average first leaf index suggests that new foliage begins to emerge in early April, on average, in southern Nebraska, which is well before the average last spring freeze. Usually, a late freeze will impact the new growth if it has begun to emerge, but it can also damage twigs and trigger fungal infections as well. If the damage is limited to new growth the overall impact to the health of the tree should be minimal as long as the tree is otherwise healthy.
Sunscald or freeze damage
Sunscald or freeze damage on thin-barked trees can also be a concern. In this case, methods such as wrapping the trunk in white plastic or using a white paint have been employed by some, but these do have drawbacks. While they do reflect the sun, they can also have a greenhouse effect and heat up between the tree and the material. Likewise, plastic is often left on the tree for too long and starts to cause other problems. In most cases, damage of this sort can be minimized by making sure trees are planted at a proper depth, and by selecting the correct tree for the site.
What can you do?
In many cases, freeze damage can take some time to show up, and there is relatively little that can be done to “treat” it. Dead branches can be pruned out, but unfortunately, we can’t control the weather. It is important to make sure that trees are planted at a proper depth, receive adequate amounts of water, and are otherwise healthy. Any additional stressors that are acting on a tree will make it more susceptible to damage during severe weather events, and less likely to recover if it does get damaged. Care should also be taken when planting trees to select species and cultivars that are proven to do well in Nebraska.
Climate Data
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30-year average (1981 to 2010) "first freeze" is October 5. |
30-year average (1981 to 2010) "last freeze" is April 27. |
| Earliest recorded "first freeze" was September 11, 1940. | Earliest recorded "last freeze" was March 27, 1925. |
| Latest recorded "first freeze" was November 7, 1900, and November 7, 1956. | Latest recorded "last freeze" was May 29, 1947. |
Citations
Lead image by Laurie Stepanek.
Labe, Z., Ault, T. & Zurita-Milla, R. Identifying anomalously early spring onsets in the CESM large ensemble project. Clim Dyn 48, 3949–3966 (2017). https://doi.org/10.1007/s00382-016-3313-2