From the perspective of the plant, "deer resistance" is not an absolute guarantee of immunity but a spectrum of survival strategies. Plants cannot run from predators, so they have evolved a range of physical and chemical defenses to deter herbivory. A plant is considered "deer resistant" when its defensive traits make it less palatable than other available options in the landscape. Deer are opportunistic feeders; when preferred, tender plants like hostas or daylilies are scarce, they may sample even well-defended plants. Therefore, the resistance of a plant like the Black-eyed Susan (Rudbeckia spp.) must be understood as a relative measure of its inherent deterrent qualities.
Black-eyed Susans possess several physical characteristics that contribute to their deer-resistant reputation. The most significant is the texture of their foliage and stems. The leaves are not smooth and succulent but are instead covered in small, coarse hairs. These tiny hairs, known as trichomes, create an unpleasant, rough, or even prickly sensation in the mouth of a browsing deer, much like the feeling of sandpaper. This hairy texture is a primary deterrent, encouraging deer to move on to more appealing, softer-leaved plants. Additionally, as the plant matures, the stems become notably fibrous and tough, making them difficult and unappealing to chew. While the bright yellow petals are soft, the prominent, dark brown central cone is dense and seed-filled, offering little nutritional reward for the effort required to reach it.
Beyond physical barriers, Black-eyed Susans employ chemical defenses that enhance their resistance. The foliage and roots of Rudbeckia species contain compounds that can be irritating or bitter-tasting to mammals. While not as potent or toxic as the chemicals found in plants like foxglove or monkshood, these compounds are sufficient to make the plant unpalatable. The specific chemical makeup is not as well-documented as in some other resistant plants, but the general principle holds: the plant invests energy in producing secondary metabolites that serve no direct purpose for its growth but are crucial for its defense. When a deer takes a tentative bite, the bitter or slightly acrid taste is a clear signal that this is not a desirable food source, leading the animal to spit it out and avoid it in the future.
It is crucial to acknowledge that a plant's defenses are not static, and under certain conditions, Black-eyed Susans can become vulnerable. The period of greatest risk is during the early spring when new, tender shoots are emerging from the ground. At this young stage, the protective hairs on the leaves are less developed, and the concentration of bitter compounds may be lower, making the fresh growth more appealing to a hungry deer. Furthermore, during times of extreme drought or in winter when food sources are severely limited, deer are more likely to overcome their aversion and browse on plants they would normally ignore. A well-established, mature Black-eyed Susan in a garden with other more desirable plants will likely be left alone, but a young plant in a barren landscape may be seen as the only available option.
From a plant community perspective, the deer resistance of Black-eyed Susans can be leveraged to protect a garden. By planting them strategically among other plants with strong deer-resistant traits, you create a defensive perimeter or an intermingled barrier. Deer rely heavily on their sense of smell and taste to assess plants. When they encounter a garden filled with the strong scents of lavender, rosemary, or sage, or the textured foliage of yarrow, Russian sage, or ornamental grasses, they are more likely to bypass the area entirely. In this context, the Black-eyed Susan benefits from the collective defense of its neighbors. Its role is not just to survive individually but to contribute to an overall landscape that is visually cohesive and effectively unappealing to deer.
