There is a language of the garden that gardeners cannot speak, messages on the airwaves that we do not receive. This language is written in the molecules of scents and colors, messages sent from flowers to pollinators, from leaves to herbivores, from one insect species to another. Every event of consequence to the gardener, including pollination, herbivory, and the control of herbivores by beneficial insects, is governed by this language.
The interplanting of nasturtiums with cucumbers to trap aphids serves as an example. Nasturtiums are a well-known magnet for aphids and thus their presence will protect the cucumber plants from serious injury by these sap-sucking insects. As the aphid population grows on the nasturtium plants, aphid-eating beneficial insects such as lady beetles, both adults and larvae, eventually appear, and since a single lady beetle can eat as many as 5,000 aphids in her lifetime, it does not take long for the aphid population on the nasturtiums to decline.
Some gardeners, fearful that the aphids might leave declining nasturtiums to feed on the cucumber plants, pull up and discard the “trap crop”, along with the attending aphids, before the lady beetles arrive. This seems shortsighted, as it may prevent establishment of the predator population in the garden. I have grown the same nasturtium plants through the summer, watching aphid and lady beetle populations seesaw back and forth, and enjoyed the beauty of nasturtium blossoms through aphid booms and busts. My only concern was to wash the aphids from the depths of the nasturtium flowers before tossing the colorful blossoms, along with sliced cucumbers, in a summer salad.
When growing space is at a premium, as it is in Marjorie’s Garden, the nasturtiums can be grown in pots, a “moveable feast” for both herbivore and predator. If things seem under control in the cucumber bed, the pot can be moved to another spot where aphids are multiplying. The mysterious communication links between nasturtiums, aphids, and lady beetles will continue to function in this new garden spot.
Annuals for Containers
Annual plants are particularly well suited to container growing, enabling the gardener with limited space to grow herbivore traps, such as nasturtiums, and to provide pollinators and other beneficial insects with sources of all important nectar and pollen throughout the year. Not all annuals, however, are equal with respect to these tasks. Many of the best annual plants for attracting beneficial insects, including pollinators, exist in two plant families, the Asteraceae (sunflower family) and the Umbelliferae (carrot or parsley family).
Annual plants in the sunflower family that are recommended for attracting beneficial insects include bachelor’s buttons, calendulas (pot marigolds), cosmos, Mexican sunflower, zinnias, and, of course, sunflowers. We have grown all of these in pots with the exception of the Mexican sunflower, Tithonia rotundifolia ‘Torch’, a four- to six-foot-tall plant that would be stunted in all but the largest container. Planted in a tub or in the ground, its large orange-red flowers will attract bees and butterflies through the summer.
Once you grow calendulas in your garden, you will always have them. Plants in pots will scatter seeds over a wide area
and the following spring their bright green strap-like seedlings will pop up everywhere. We leave them to grow along the bed edges and transplant those that are in the beds to pots, or to empty spots along the beds edges. Wherever they are grown, calendulas are a favorite of the small native solitary bees.
Tall sunflowers are difficult to grow in pots, but there are dwarf varieties that are short and bushy, ideal for containers. Be sure to select a variety that produces pollen, rather than one of the pollen-less florist varieties, since pollen is one of the rewards that sunflowers offer visiting
pollinators. In Marjorie’s Garden, sunflowers are foraged by bumblebees and the smaller solitary bees in search of both nectar and pollen.
Annuals in the Umbelliferae that attract beneficial insects include fennel, chervil, coriander, cilantro, and dill. Two biennials in this family, carrot and parsley, are typically grown in the garden as annuals, but when they are left in the ground in mild climates and heavily mulched over the winter, come spring they will produce “umbels”, broad flat-topped clusters of tiny white flowers typical of the family. In colder climates, carrot roots must be dug in late fall, stored over winter, and replanted in spring to flower. Like those of the wild carrot, Queen Anne’s lace, the flowers of garden carrots and other Umbelliferae attract a wide array of beneficial insects, including pollinating bees, beetles, and flies, as well as predacious hoverflies and
parasitoid wasps. The later are tiny wasps that lay their eggs on the larval or adult forms of herbivores such as the tomato hornworm. The wasp larvae burrow into the host’s body and consume it as they grow.
Several other plant families contain annual species that attract beneficial insects. The mint family, Lamiaceae, includes the culinary herb basil which can be allowed to flower in the garden to attract pollinators. The mustard family, Brassicaceae, home to an entire group of garden vegetables including cabbage, broccoli, and cauliflower, also includes sweet alyssum and annual candytuft, two popular garden plants with flowers that attract adult hoverflies, the larvae of which eat aphids and other garden herbivores. Sweet alyssum nectar is also a favorite of tachinid flies, a large group of parasitoid wasps that lay their eggs on the larvae of garden herbivores. The larvae that hatch from these eggs consume the herbivore larvae.
An Insectary of Potted Annuals
For those gardeners with plenty of space, the insectary, a section of the garden planted to attract pollinators and other beneficial insects, is often a bed or border near the vegetable garden where the services of those insects are most needed. Planted with a variety of herbaceous plants, both annual and perennial, and planned to ensure offerings of nectar and pollen through the garden year, the insectary will sustain populations of native bees, butterflies, hover flies, parasitic wasps, and other beneficial insects, ensuring that they will be there when needed to pollinate garden crops and control populations of herbivores.
Not every garden, however, affords the space for an in-ground insectary. A tomato plant growing in a raised bed on the urban patio still needs the pollinating bumblebee and the parasitoid wasp predator of the tomato hornworm. What can the gardener with limited space do to keep these and other beneficial insects at hand? Think pots, containers filled with a variety of plants sufficient to keep nectar and pollen at hand from early spring to first frost. The gardener creates this insectary anew every year, leaving the rest to the chemistry of the garden.