As Spring Winds Blow, Sap Flows

When warmer winds begin to blow east over the Hudson River, it’s a welcome signal that spring is on the way. Trees have laid dormant all winter; dormancy allows them to survive freezing temperatures by consolidating water and nutrients in their roots. As we progress through the season, they begin to wake up in succession, greening up as they go.

The end of dormancy is signaled by two phenomena: bud break and sap rise. Bud-break signifies the emergence of immature leaves or flower buds. Sap rise describes the flow of stored sugars and water from the tree’s roots up into its canopy. It is still unclear to scientists which comes first, sap-rise or bud-break. It seems to vary by species—another one of nature’s mysteries. While bud-break signals gardeners that the window for dormant pruning is closing, sap-rise still affects late-season pruning. That’s because every cut creates an opening for sap to weep out.

Knowing when species break dormancy and how they react to pruning helps gardeners make precise cuts at the right time, preventing disease spread and damage to the trees.

Let’s take an example in our gardens. Hollies (Ilex spp.) react well to late-season pruning. Shown at the top of this email is a snapshot of Wave Hill Gardener Christopher Bivens earlier this month, giving some strategic shaping to Ilex ‘Dragon Lady’ at the Kerlin Overlook.

I caught up with him during a trimming moment. “I'm making lots of small cuts to give this holly a columnar shape,” was his explanation of what looked like an activity that clearly required precision. This session of late-season pruning gives the holly tree a refined appearance, adding to the structure of this formal garden area that looks west toward the river form a small plateau.

In fact, trees are generally invaluable to the structure of our garden areas. Many of Wave Hill’s trees predate Wave Hill’s formation as a non-profit in the mid-20th century. Some even predate the formation of the private estate in the 1800s! Celebrate Wave Hill’s great trees over the next few days: it’s Arbor Weekend!

Jess Brey,
Ruth Rea Howell Senior Horticultural Interpreter