While Mike Hynson and Robert August’s only tangential connection to agroforestry was perhaps in the old-school wooden surf boards they used, the ‘endless summer’ I chase is more of a reprieve from a summer-ending frost descending on the garden. And agroforestry practices, specifically through strategically placing a sheltering canopy of trees or shrubs, can be a great benefit to those of us in the north or the way-down-south who roll with the rhythm of the changing seasons.
Living with the four seasons means planning your production in the frost-free period between the last of winter’s fury in the spring and the first frosts of autumn. This window of opportunity sets the range of crops that can be grown in temperate and boreal climates. This limitation is expressed particularly in the fall when many crops are in the final stages of ripening; an untimely early frost can undo an entire year’s work, or at minimum, lower crop quality and yield. And as is if often the case, an early frost can be followed by 3, 4 or more weeks of good growing conditions. Lengthening out the growing season by month or more is possible by ‘surfing’ over the early radiative frosts that can form when daytime heating is not sufficient to balance the heat loss on clear, cool autumn nights. Agroforestry plantings have the advantage of trapping and releasing heat, keeping the fall frosts away, so the crops below can live to grow another day.
Trees and the woody stems of tall shrubs, being largely composed of water, are very efficient at absorbing the radiant energy from the sun. This creates a natural heat reservoir that emits long-wave radiation into the surrounding area. The tree’s canopy also blocks heat loss from the ground to the sky, reflecting some of this wayward energy back towards the ground. These forces acting in concert build and retain higher night-time air temperatures and nullify radiative frosts that occur when heat escapes from a crop’s surface to an unobstructed sky. This moderating influence in cold climates can significantly extend the growing season by reducing or eliminating late spring or early fall frost damage. Research I conducted in central Alberta demonstrated a partial cover of trembling aspen could extend the frost-free growing period by an impressive 41 days in the northern prairie-parkland zone and by more than two months in the boreal where frosts are not unknown in August.
Plastic tunnels, wind machines and portable heaters can also serve to drive away light frost events but they don’t share the other benefits of agroforestry plantings: sequestering carbon, creating wildlife habitat, other sheltering benefits against wind and water erosion, and producing other useful and valuable products (wood, nuts, fruit, boughs, etc.). And besides that, I think that I shall never see a poly-vinyl chloride shelter as lovely as a tree.
When the cold temperatures are sustained for longer periods or a below-zero air mass engulfs your area, agroforestry systems are in the same proverbial boat as conventional plantings. Agroforestry can only delay the onset of winter conditions; it can’t prevent them. But a well-planned agroforestry planting can buy you some valuable additional growing days. So, ‘catch a wave’ and extend your growing season with agroforestry.