Mast Year

For oak trees, mast year means a bumper crop of acorns. (Photo: Shutterstock)

Carol O'Meara, Colorado State University Extension

Carol O’Meara, Colorado State University Extension

LONGMONT – My neighborhood is filled with curious people, folks who pull out lawn chairs to watch the goings-on in my yard or devise reasons to stroll by for a closer view. I don’t blame them; things at my place are usually on the edge of chaos, what with me blundering about with power tools, loudly admonishing plants, or arguing with the squirrels.

The squirrels sass back and now my oak tree has gotten in on the action by producing a copious amount of acorns. The nuts are falling, covering the grass. Mowing has become a hazardous activity, so to prevent serious injury, the acorns have to be cleaned up. But there are hundreds of them and efficiency is key; plucking them up by hand would take me into winter. In the face of this, I did what any reasonable gardener would do and reached for technology to speed things up.

Given the size of the acorns and odd twigs, our shop vac was put into action, triggering a chain reaction amongst the neighbors as they settled in to watch the spectacle of my clearing the yard from nuts. I don’t mind a few people sitting in lawn chairs but the vendors selling T-shirts and popcorn were a bit much. You’d think they’d never seen someone vacuuming the lawn while being pelted with acorns by enraged squirrels before.

Our tree is a burr oak, a member of the white oak clan. White oaks produce acorns in one growing season while red oaks produce them in two. Late spring frosts can blast the flowers of white oaks, resulting in poor acorn production by fall. Red oaks are less affected by this in the same growing season, but will have fewer acorns the following year.

In years with ideal temperatures, water, and pollination oaks experience a “mast year.” A mast year is when the tree produces a bumper crop of nuts, far more than in other years. Mast years occur every four to seven years, but their timing is still a mystery, often being related to ideal weather or genetics. Because it takes a lot of the tree’s resources to produce all the nuts, several years are needed to recover.

Mast years occur across areas, with some, but not all, trees bearing heavy loads of nuts. This synchronicity of oaks having mast years ensures that there will be an abundance of acorns in the region. Researchers believe it is a way for the trees to ensure enough acorns escape the greedy clutches of squirrels and other animals and germinate into seedlings.

White oak acorns tend to be selected by wildlife more than red oak acorns because they’re more palatable; they have less tannins so are less bitter and more digestible.  Nutritional needs of wildlife vary with the seasons and in fall their concern is energy for migration, hibernation, or as stored food for winter. I’d love it if my squirrels migrated, even as far as the neighbor’s yards, but it seems they prefer my place.

The acorns produced by my oak are part of what they love, along with the black walnuts in my back yard and the peanuts someone is feeding them. In this, a mast year, the squirrel’s bounty knows no bounds. Nuts are very high in carbohydrates, making them an important energy source especially in the colder months.

It’s good to know that oaks  are plagued enough by squirrels that they’ve developed a way to outsmart them. Don’t worry – despite my vacuuming, there are plenty of acorns to go around, so the squirrels will stay fed all winter.    

By Carol O’Meara. Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development. For more information contact Colorado State University Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, e-mail [email protected] or visit