The last plant to leaf out at the Arnold Arboretum: Makino Rhododendron

Post by Richard Primack

Every spring for the past three years, we have been monitoring the leaf out times of over one thousand species of trees, shrubs, and vines at the Arnold Arboretum of Harvard University. We try to survey the entire collection at least once a week, recording when the first flush of new leaves are produced for each species.

The work is important for ecosystem research in determining when the growing season begins, when new leaves are available as food for animals, and when plants start to absorb atmospheric carbon dioxide. This work is also important in determining the ability of plants to respond to a warming climate. And lastly, the variation among species in leaf out is a rich area of natural history that has been surprisingly neglected.

At the Arnold Arboretum in 2013, the first plants began to leaf out in mid-March. These were mainly species of honeysuckles (Lonicera), gooseberries (Ribes), and privets (Ligustrum). The last species did not leaf out until 2 to 3 months later, and these were mostly species of pine (Pinus), fir (Abies), and spruce (Picea), along with a few evergreen rhododendrons. The last species to leaf out at the Arnold Arboretum in 2013 was the Makino Rhododendron (R. makinoi), which did not produce its new flush of leaves until mid-June, three months after the first species.

Rhododendron makinoi, the last plant species to leaf out at the Arnold Arboretum. 

(This photo was taken in mid June of 2013) 

We are currently cooperating with botanical gardens from around the world to monitor leaf out times on a larger scale. Our goal is to determine the relative advantages and disadvantages of early and late leaf out times.

For further reading, see http://arnoldia.arboretum.harvard.edu/pdf/articles/1976.pdf