Now that spring is here, Central Park’s trees are beginning to display their first hints of green – a process known as leafing out. In addition to marking the end of winter for New Yorkers, the emergence of new leaves signals the beginning of the growing season for the Park’s woody plants. Leaf-out dates vary not only from species to species, but also from year to year depending on climate. The study of the cyclic phenomenon of leafing out has grown world-wide as ecologists connect leaf-out dates to other issues, such as climate change, although horticulturalists have been paying attention to this for years. For us, as Central Park’s caretakers, leaf out is an indication that plants are leaving hibernation, just like Park visitors are. This sign of spring means that horticulture and irrigation in the Park will need to be adjusted for the growing to come.
Recent warm weather has triggered many plants to leaf out and flower in Central Park. Magnolias and redbuds are in bloom and you can see gingko leaves unfurling. However, we’ve had a few cold days as well, which can have an impact on these new buds, flowers, and leaves. In New York City, our last frost date is predicted to be April 20, according to the National Climatic Data Center. New growth can be susceptible to frost and freezing damage during this time, when cold winds and below freezing temperatures are still possible. The extent of potential frost damage depends on temperature, plant species, exposure, and the plant’s stage of growth. Some buds could be aborted by a frost (i.e., they drop off and don’t flower), but this depends on the particular plant’s exposure to the elements. Often, if tree leaf buds sustain frost damage, new leaves will take their place. If perennials get frost damage, their crown (or growing point) will send up new growth in a matter of weeks. Plants know what to do to recover!
Opened buds, particularly flowers, don’t always do as well in a frost. This magnolia was bitten by Sunday night’s frost and its flowers have been damaged – compare its blooms with the undamaged blooms on the tree in photo 1.