Early Warm Weather May Cause Problems for Ornamentals Kyle Daniel, Nursery and Landscape Outreach Specialist, Horticulture and Landscape Architecture Rosie Lerner, Consumer Horticulture Specialist, Horticulture and Landscape Architecture Lindsey Purcell, Urban Forestry Specialist, Forestry and Natural Resources Larry Caplan, Vanderburgh, Extension Educator-Horticulture Kris Medic, Purdue Extension Educator, Bartholomew, Community Development, Agriculture & Natural Resources Nikky Witkowski, Lake, Extension Educator - Agriculture & Natural Resources Steve Mayer, Marion, Extension Educator Horticulture Figure 1. The National Phenology Network Spring Leaf Index Anomaly March 7, 2017. Most of Indiana is at least 20 days ahead as compared to normal spring bud break. https://www.usanpn.org/data/spring We are all aware of the progression of the early warm weather, but to visualize the amount of areas impacted really underscores the point (Fig. 1). The National Plant Phenology Network updates the current status of spring and details their methodology in defining spring: https://www.usanpn.org/data/spring.
Most of Indiana has accumulated greater than 900 chilling hours (Fig. 2), which fulfills dormancy requirements for most ornamentals. Southern Indiana has callery pear in bloom (Fig.3), multiple species of maple in leaf, and many other species currently breaking bud, while northern Indiana is about two weeks behind (Table 1). Temperatures play a key role in de-acclimation from dormancy in the spring. When an extended time of warm temperatures occur, and chilling hours have been met, bud break begins to occur, as well as the vascular system becomes more active transporting water and other assimilates. If temperatures cool, the bud swell can be suspended, but for many species, especially in southern Indiana, buds have already broken past the point of suspension. When bud break occurs followed by cold temperatures injury can occur, such as bud kill. This could pose problems with apical dominance, leading to accessory bud break. Since the vascular system of the plant is active, bark splitting or cracking may also occur on thin barked trees if a cold period occurs after extended warm temperatures. Unfortunately, there appears to be a significant cold front move through Indiana late this week and into the weekend with low temperatures ranging from 23 0 in Evansville to 17 o in Fort Wayne Saturday morning. With many plants in bloom in southern Indiana and breaking bud in northern Indiana, damage can be expected. There aren t many options for protection on most plants, especially on a large scale. At this point it is too early to determine whether any damage will occur, but you should be inspecting plants as bud break occurs. Damage may not be visible on the outside of the plants, but necrotic (dead/dying) tissue will be evident in the vascular tissue. However, the good news is that this phenomenon is typically not lethal for most established woody plants. Best management practices suggest patience and good assessment. Be certain the new growth is dead by scratching the thin-layered bark to assess the cambial tissue. Pruning out dead branches of trees and shrubs may be required this spring after damage has been assessed, but delay pruning as much as possible as to not encourage further budding. If you have any questions, please contact Kyle at daniel38@purdue.edu. Figure 2. Chilling hours accumulated by the Midwest Climate Watch Vegetation Impact Program. http://mrcc.isws.illinois.edu/cliwatch/special_topics/agriculture.html#chillhours 2
Figure 3. (Pyrus calleryana) in bloom in southern Indiana. 3
Figure 4. Elm buds beginning to swell due to warm temperatures. 4
Bartholomew Vanderburgh Lake Tippecanoe Marion Red Maple budding sporadically Peony emergence Silver maple past flower stage sporadically Multiple species of bulbs emerged pushing and saucer breaking bud Peaches half-inch green to pink nearing bud break Saucer and star breaking bud Witch hazel Apples swollen bud Witch hazel Vegetative growth on roses has begun Willow leafed out, lilac swollen buds Vegetative growth on roses has begun Redbud at bud swell Table 1. Current plant stages at various locations around Indiana. 5