What Causes Buds to Open in Spring?
Spring has finally sprung! Not only are crocuses, day lilies and daffodils starting to show up in our gardens, but many trees are also budding. It is a wonderful sight, giving us hope for longer and warmer days to come. Did you ever stop and think what causes trees to bud in the spring? Many think it is the length of day or the warmer temperatures, but the most important environmental cue for spring bud break is actually cool temperatures!
Dormancy (not growing or not active) is important for all tree species. It allows them to avoid the harsher conditions of winter. There are two stages in dormancy: endo-dormancy and eco-dormancy. During endo-dormancy, plants will not grow, even during warm conditions, due to factors inside the plant that inhibit or stop growth. This stage starts when winter begins and prevents trees from budding during an unseasonably warm stretch during the winter. During endo-dormancy, trees start to track the time spent above freezing, called chilling units. Temperatures between 40 – 50 degrees Fahrenheit are most effective. The duration of exposure needed to break dormancy varies by species and location, ranging from 500 – 1,500+ hours.
Once the chilling period is completed, usually in January in the Mid-West, eco-dormancy begins. This is when the conditions are not quite right, usually too cold, but the tree is ready to grow. When the temperature reaches the mid-40’s or warmer, growth begins and the buds start to break. These buds were formed the previous summer and spent the winter dormant and protected under bud scales. Now the young shoots break through and begin unfurling the leaves for this summer!
The new tissues coming out in the spring are very susceptible to disease, such as fungi, bacteria, phytoplasmas, and other pathogens. Spring bud break is the perfect time to treat your plants that have had problems with disease in the past. A pathogen from last year will probably be there again this year. The most threatening diseases in spring are:
- Anthracnose on ash, maple, and oak
- Apple scab
- Fire blight on hawthorn, pear, crabapple
- Needle casts on pine, Douglas fir, and spruce
Spring is also a good time to look for things that might have occurred over the winter. There are many kinds of winter injury that may have effected your trees. Temperature fluctuation, extreme low temperatures, wind, and animals can all cause damage. Here are some damages to look for:
- Frost Cracks – deep longitudinal cracks from cold temperatures
- Winterburn on Evergreens – browning or scorched leaf tip from wind
- Spring Freezes – new leaves become flaccid and wither after a sudden hard frost
- Salt Damage – browning of evergreens, leaf scorch, and branch die back
- Girdling by Animals – mice and rabbits feed on young tree bark in the winter
Conservation Corner is a weekly article produced by the Forest County Land &Water Conservation Department. For more information contact Steve Kircher, County Conservationist-Land Information/GIS Director at 715-478-1387 or by e-mail at .