Why do some plants leaf out in spring before others?
C. James McGrath, Bristol, RI
Emerging foliage can indicate when it's time to plant.
Photo/Illustration: Haley Aselin Graves
Tolly Beck, a horticulture educator for the Cornell Cooperative Extension and an instructor at the
New York Botanical Garden
, replies: Hereditary genetics plays a major role in determining when a plant will begin to leaf out. Inherited genes are those that are passed along in plants from one generation to another. Certain genes dictate the conditions necessary for leaf buds to begin active growth in a particular plant. Each plant, therefore, has a predetermined set of conditions that signal it to leaf out.
One of the earliest trees to leaf out is the Ohio buckeye (Aesculus glabra, Zones 3–7). Leaf emergence in this plant occurs from late March to early April, with flowering occurring in mid-May. A short period of dormancy initiates its early leaf growth. Later leaf emergence is seen in the Eastern redbud (Cercis canadensis, Zones 4–9), where flowers are produced from March to April before the leaves begin to emerge. The Eastern redbud requires a longer period of warm days to initiate leaf growth. These are examples of inherited characteristics that will continue to be found in each generation of these particular trees.
The date at which plants commonly leaf out can be affected by the weather and other environmental factors. Spring temperatures, winter chilling, cloudy weather, the previous autumn’s conditions, and soil moisture all have an influence on when a plant will leaf out. Trees will often leaf out earlier when spring temperatures are warmer than usual.
The study of the relationship between certain biological events (like leaf emergence) and climatic conditions is a science called phenology. By observing specific phenological events over many years, certain reliable correlations can be made. One such correlation is that when leaves first appear on common lilacs (Syringa vulgaris and cvs., Zones 4–8), it is the correct planting time for beets, carrots, cole crops (belonging to the Cruciferae or mustard family), lettuce, and spinach. This correlation is made because the conditions that are necessary for the common lilac to produce its first leaves are the same conditions necessary for optimal initial growth for these specific crops.
With the progression of genetic adaptations and changing climatic conditions, there will likely be some interesting developments in the timing of leaf emergence. If a warming trend consistently leads to earlier leafing out, there may be a shift northward for many plants. While this sounds like a wonderful situation for gardeners, it may create problems for pollinators and bird species that may not be able to adapt at the same rate.