In recent weeks I have used my column, “Gardening in Tune with Nature”, to focus readers’ attention on attracting the pollinators and other beneficial insects that are so important to success in growing many vegetables and small fruits. The best strategy is to keep these insects in your garden throughout the year with a constant supply of pollen and nectar and a variety of winter nesting habitats. In this week’s column, I recommend a handful of native flowering shrubs and trees that are both ornamental and ecologically functional, plants that offer pollen and nectar to foraging insects in early spring and late summer, times when the vegetable garden itself does not have much to offer these important insects.
Summersweet Clethra (Clethra alnifolia)
If I had space in a small garden for only one woody plant, I would grow summersweet. I’ve always grown it and frequently written about the heady perfume of its summer flowers and its unique brassy yellow autumn leaves, but its status as a “must have” plant for the garden insectary was secured last August as I stood among a colony of the cultivar ‘Hokie Pink’ flowering in Marjorie’s Garden. I recalled the summer day ten years earlier when we knocked each of the five small plants out of its one-gallon pot and planted them around the bole of the old white pine growing off the porch steps. A decade later, as I twisted my way into head-high branches crowded with racemes of soft pink flowers, it was impossible to say where one plant stopped and another took over. Even a chipmunk has a hard time weaving through the interlaced stems to reach the base of the pine.
Carefully moving branches aside, I positioned my legs and those of my tripod in the middle of the summersweet colony and waited for the wake of my disturbance to settle. Soon the spikes of flowers, only a few inches from my nose, were crawling with bees, wasps, hoverflies, beetles, and several insects whose images remain on my computer desktop, waiting to be identified. Bumblebees of all sizes and colored markings outnumbered all other insects. A lonely honeybee joined the symphony of buzzing, along with several small native bees, some metallic green, others gray or black. Two distinctly different hoverflies tormented me with their inability to settle down long enough for a photograph, but a tachinid fly obliged, stopping in its frenetic foraging for nectar long enough for me to get a decent shot. (Both hoverflies and tachinid flies are predators of herbivores such as aphids and various leaf-munching caterpillars. Hoverflies feed on nectar and pollen as adults, but their larvae feed on other insects. Tachinid fly females lay their eggs on other insect adults or larvae. When these eggs hatch, the tachinid larvae bore into the host’s body and slowly consume it.)
In terms of diversity of insects attracted to a single plant in bloom, nothing in my experience compares to what I witnessed on that August afternoon. Through the tangle of summersweet branches I could see the vegetable garden a hundred feet away, the bright orange squash flowers beckoning. When a bumblebee in my viewfinder took wing and disappeared, I knew where it was heading.
Highbush Blueberry (Vaccinium corymbosum)
In early June, long before tomato blossoms appear, the highbush blueberries in Marjorie’s Garden are in full flower and swarming with bumblebees. These plants grow in beds at the heart of the vegetable garden, near the tomato plants that will need the unique “buzz pollination” services of these same bees a few weeks down the road.
Tomato plants with flowers pollinated by bumblebees produce more fruits than plants bereft of bumblebee services. While tomatoes are capable of some self-pollination, more pollen is released when the bumblebee grasps the flower with her legs and moves her flight muscles rapidly, causing the anthers to vibrate, dislodging the pollen far more effectively than wind. It pays to keep the bumblebees happy until tomatoes flower.
Red-vein Enkianthus (Enkianthus campanulatus)
You will not often find me recommending a non-native plant for attracting pollinators,
but red-vein enkianthus is the exception. A member of the same plant family as blueberries, enkianthus flowers are another favorite of bumblebees. Like those of highbush blueberries, these pendulous white flowers, each petal marked with red veins, precede those of tomatoes, so an enkianthus growing near the vegetable garden will enhance pollination of tomatoes. In addition to its June display of flowers, this Japanese shrub or small tree, depending on the disposition of the pruner, will grace your garden with beautiful autumn foliage and lovely winter form.
Mapleleaf Viburnum (Viburnum acerifolium)
We have been growing mapleleaf viburnums for over ten years, enjoying their spring flower display and the unusual colors of their autumn leaves, salmon pink with splashes of dark purple. They survived the scourge of viburnum leaf beetles and now flourish in Marjorie’s Garden with little effort on our part. Their chief pollinator seems to be a tiny fly that appears in mass on each flat-topped cluster of tiny white flowers when the pollen is ripe, and just as quickly disappears when the pollen is gone. I include them in this list because their flowers also attract hoverflies in search of pollen. The larvae of these hoverflies may well go to work in the vegetable garden, eating aphids and other herbivores.
The four plants mentioned above are by no means the only plants that attract pollinators and other insects to Marjorie’s Garden. The Allegheny serviceberry (Amelanchier laevis), a small tree with lovely white flowers in May, depends on bees and flies for pollination, as do the winterberry hollies (Ilex verticillata) scattered about the garden. And the staghorn sumacs (Rhus typhina) that grow on the fringes of the garden are important plants for bees and serve as larval hosts for butterflies and moths, including the luna moth, the largest and most beautiful moth in our garden.
Remember that in addition to planting for pollinators and other beneficial insects, it is equally important to provide suitable nesting habitat if you want to keep these all-important creatures in the garden. To provide nesting sites for bumblebees, leave a rotting stump or two in the garden, or an old board or upturned flower pot on the ground. Some species of native bees require patches of bare compacted soil for their underground nests, while others will accept wooden nest boxes in lieu of old wooden fence posts with holes drilled by beetles. Provide all of these nest options in order to maximize the diversity of garden pollinators.