Phenology: When Trees Flower, And Much More

As spring unfurls we see tree phenology in action. What early bloomers are catching your attention? I saw (and smelled) a beautiful star magnolia in Shepherdstown yesterday. Red and silver maple flowers are popping all over the place. If any of you are DC commuters, then you may have seen magnolias, cherries, ornamental pears and perhaps early redbud and dogwood blooms. We have all of this natural artwork and more to look forward to here in the eastern panhandle in coming days.

Of course, phenology covers more than simply the joy of flowers. It encompasses all physical displays of growth and reproduction, and awareness of phenology provides us with more benefits than simple aesthetics. Here are just a few examples of the usefulness of knowing which trees are doing certain things at a certain time:

Stinko ginkgo control – If you live on a ginkgo block in DC with female trees as a part of the mix, you will be grateful that the Urban Forestry Administration waits until the flowers are just emerging before they begin their spraying programs. Otherwise there would be no effect.

EAB injections – Trees 101 is going to be treating ash trees for the emerald ash borer this spring and summer. The most effective insecticide is itself most effective when injected early during the tree’s active growing season. The vascular system is most active and transpiration is at its peak at this time. All the information suggests to me that mid-May is the right time this summer, but how can I really be sure?

Protecting bees – Even natural sprays to deal with tree/shrub pests can dramatically impact bee activity if applied while the target plant is in flower.

There are a few organizations that are taking advantage of the internet and ‘crowd sourcing’ phenology information from around the country. They ask volunteers to pay attention to trees that they regularly come across and enter any observed phenological events in their data base.  The result is a map that shows how the timing of these events varies across the country and how we see variation from year to year even in the same location. This information can be referred to for uses like those listed above, but perhaps more importantly it can, over time, inform us about the impact that climate change is having on our tree populations. Trees don’t have legs and cannot migrate to keep up with the warming trends we are experiencing. The implications of this fact are overwhelming but this kind of ‘crowd sourced’ phenology can at least provide some concrete observations of the phenomenon.

The Education Department at Casey Trees in DC is coordinating a District-based phenology project. They are the ones who got me thinking about this as a blog topic, actually. If interested, check them out at They are working through a larger effort conducted by the US National Phenology Network (, and there is another similar effort called Project BudBurst ( All of these groups emphasize the value of the data AS WELL AS the value of the stewardship and education that is nurtured through keeping a close eye on your tree population. Will you be the first to log the flowering of paw paws along the Potomac? Have fun!