Illustration by Adelaide Tyrol
Forests would be a lot less interesting and not nearly as beautiful without ferns, wildflowers, shrubs, understory trees, and other small plants. But as the canopy closes in a forest, the amount of light that filters through to the ground diminishes greatly, slowly starving all but the most shade-adapted residents of the lower layers.
Hobblebush, a sprawling shrub that has beautiful, showy white flowers in spring, succeeds well at growing in deep shade, using several strategies that keep it from having to declare photosynthetic bankruptcy, despite having very little regular solar income.
Just like the wildflowers called spring ephemerals – the ones that make use of the flood of sunlight available in March, April, and early May before trees in the canopy leaf out – hobblebush leaves open early, sometimes beginning when there is still snow on the ground. This allows them access to high light levels for two to four weeks, ending when overstory leaves have fully expanded. This early leaf-out, according to a study by D. S. Canny, M. P. Ayres, and J. J. Ruel published in the Canadian Journal of Forest Research in 2000, increases hobblebush’s annual amount of photosynthesis by 40 percent.
The main reason for leaves not to open early is frosty nights. It’s worrying to see newly unfolded leaves killed and blackened by frost, as this loss must be quite costly to a tree or shrub. It’s an expense that hobblebush avoids by having enough antifreeze agents in its leaves to reduce the freezing point to 5˚C below freezing – a temperature that is 1 to 2 degrees lower than the freezing point of yellow birch, whose leaves were previously considered to be the most cold tolerant of the woody plants.
In addition to being able to get by at an unusually low thermostat setting, hobblebush is thrifty. All plants expend energy all the time, day and night, just to perform basic functions, and every plant has a “light compensation level,” the amount of light that is needed to break even. Light above this level will allow for growth or the accumulation of stored energy. At less than the light compensation level, the plant can’t pay the rent. Although after leaf-out there is only 10 percent as much light in the understory as there is in the canopy, the hobblebushes studied made net gains every day through the summer, except when there was continuous thick cloud cover. At night, hobblebush is similarly penny-pinching, maintaining a low respiration rate compared to other plants.
Sometimes the success that these adaptations have brought to hobblebush is not appreciated by those who happen upon it. It is called hobblebush (and sometimes witch hobble) because the ends of its branches droop, become embedded in the ground, and take root, creating a nearly impenetrable tangle. At times, dense hobblebush thickets may interfere with regeneration of desirable tree species.
Though it survives by being opportunistic and parsimonious, in appearance it is extravagant. The flowers are in flat-topped clusters, 3 or so inches across, with a rim of very showy petals. The flowers are followed by fruits that change in color from red to dark purple in late summer. The heart-shaped opposite leaves are sometimes 8 inches long – huge for a lowly shrub that usually is less than 5 feet high.
Moose and deer feed on hobblebush, especially in winter, and the plants that they have browsed are stunted and develop odd-looking leaves. In Pennsylvania, where there are 30 deer per square mile statewide, hobblebush has declined significantly throughout the state. Once very common, it has been completely eliminated by deer in a 4,000-acre old-growth forest that was surveyed for many years.
The fruits are known to be eaten by pine grosbeak, redpoll, cardinal, red-eyed vireo, ruffed grouse, turkey, robin, starling, gray-cheeked thrush, hermit thrush, great crested flycatcher, cedar waxwing, and pileated woodpecker. White-footed mice, chipmunk, black bear, red fox, eastern gray squirrel, and skunk also eat the seeds. If there are few seeds left, it may not matter that much, for hobblebush almost always reproduces from sprouts or from the new plants that are formed when the branches take root.
Cool, moist woods and shaded ravines throughout temperate eastern North America are made particularly beautiful by hobblebush in autumn, when the large leaves slowly turn purple and then yellow before falling to the ground.