Guest post by Sarah Flanders
Keystone Species are plants that support the most species of insects. These plants are not only beautiful, but they are (or used to be) abundant in Western Pennsylvania, and without them, you cannot restore a functioning ecosystem. They are the logical choice to start a native plant garden.
Some insects are specialists and others are generalists. Specialist species depend on one primary species for survival. A single species of plant may host a few or up to hundreds of different insect species that have evolved along with them to require them for one or all of their life stages, from caterpillar to adult. For example, although an adult monarch can feed from many different types of flowers, it only lays eggs on the milkweed, because the caterpillars must eat milkweed leaves to survive. About a third of our native bees only feed on the pollen of a certain species or small group of species of flowers. (Honeybees, in contrast, are non native generalists, which is why they are sometimes called “livestock” – animals that can be shipped for pollination of food crops around the world).
Keystone species are so important to the local food web because they can host the most types of insects. Before we consider flowers it’s important to note that trees provide more biomass in the garden and a Keystone tree tends to support more insects than a Keystone plant. In our area, the top large host tree is White Oak, which is believed to support somewhere between 500 to 1000 species of caterpillars. The top smaller host trees are American Plum and Chokecherry, which support two to three hundred caterpillar species each. A keystone shrub is Northern Highbush blueberries and other native blueberries, which also support two to three hundred caterpillars.
The top Keystone forbs in Western Pennsylvania are goldenrod (genus Solidago) asters (genus Symphotrichum), and Sunflowers (genus Helianthus). Interestingly, all are part of the Asteraceae family. Goldenrod hosts more than 100 species of caterpillars.
All of these flowers are beautiful in the garden, but can look tall and weedy in smaller spaces unless care is taken to plant around their feet. In general, chopping flowers in spring controls height and curbs their tendency to flop and lose lower foliage later in the season. In addition, there are many subspecies of each genus, and some of them are more compact.. Here are some that are particularly useful for the garden.
Goldenrod
Early Goldenrod is named for the fact that it blooms as early as June, instead of August or September. It is a hardy species that can grow in sun or shade and in different types of soil. It produces bright yellow plumes of flowers and reaches about one to 4 feet tall. The fact that it blooms early means that it can be a companion to plants that bloom earlier in the summer.
Showy Goldenrod is a tall (1 to 6 feet) and late-blooming species with particularly showy and thick plumes of golden yellow flowers. It thrives in full sun or part shade and does well in dry soil. It is considered less aggressive than some of the other species,as it has a fibrous root system, not a rhizomatous one, and grows slowly in clumps.
Gray Goldenrod is remarkable for the slightly more scattered appearance of the late, bright yellow blooms. The entire plant is attractive for the sturdy, grayish green foliage and vase-like shape. It tends to reach only about 2 to 3 feet tall and does well in poor, dry, even gravelly soils in sun or shade.
Zigzag goldenrod is a smaller, more delicate, shade garden plant with scattered blooms that form a zig zag pattern. It is only one to three feet tall and brightens a shady woodland area.
Asters
Asters are similarly varied. They bloom in many shades of purple and white. The New England Aster has the classic showy, tall, bright purple blooms which catch the eye. Aromatic Aster (Symphotrichum Oblongifolium) is also quite useful as a colorful companion to taller flowers due to its low mounding habit and abundant blueish purple blooms in August to October. Aromatic asters reach one to three feet and spread (non aggressively) via stolons, and they can tolerate sun or partial shade.
A third keystone species, Sunflowers, are beloved, cheerful plants that host insects and birds alike. We have several different native types, with a variety of yellow flowers about the size of a large daisy. Commercially important sunflowers that produce the seeds we eat are not strictly native, because they came from the Western part of the US and have been hybridized for better seed production. But as they are easy to grow, about 6 feet tall, with huge blooms,sturdy stalks and bountiful seeds, they are great for attracting birds, delighting children, and creating structure and focal points in a garden.
All three of these species, goldenrod, aster, and sunflower, are easy to grow and produce abundant seeds to feed songbirds and for you to share. All but the commercial-type sunflowers should be sowed outside in winter so that they can go through a couple of months of cold stratification.
Written by Sarah Flanders