The popular garden plants in the genus Bidens, commonly known as Spanish Needles, are cheerful and prolific bloomers sought after by gardeners. These members of the daisy family (Asteraceae) are prized for their bright, long-lasting flowers and vigorous growth habit. For anyone gardening in an area with a large deer population, a common question is whether these attractive plants can withstand browsing pressure. The general consensus is that Bidens is a species deer usually avoid.
General Resistance Status of Bidens
Bidens is classified as a deer-resistant plant, meaning it is rarely a preferred food source. This natural defense stems from the plant’s unique internal chemistry, which renders the foliage unpalatable to most browsing animals. Deer are highly selective foragers, using smell and taste to distinguish between desirable and undesirable food sources. They generally bypass plants that possess strong, unpleasant flavors or tough textures.
The genus Bidens produces an array of secondary metabolites that function as natural deterrents. These phytochemicals include compounds such as polyacetylenes and flavonoids. These compounds imbue the plant tissue with a distinctly bitter or strong flavor profile that is repulsive to deer, even when the plant offers substantial green mass. This chemical profile ensures that deer will instinctively choose other available forage before consuming Bidens.
In addition to the chemical defense, the texture of the foliage in many varieties, such as the finely dissected leaves of Bidens ferulifolia, does not offer the soft, easy-to-digest browsing experience that deer favor. This combination of undesirable flavor and texture provides a robust two-part defense against browsing behavior. This inherent resistance makes Bidens a reliable choice for planting in landscapes where deer are a regular occurrence.
Factors That Compromise Deer Resistance
While the natural resistance of Bidens is significant, it is not an absolute guarantee against browsing, as external pressures can override the plant’s chemical defenses. The most common factor compromising resistance is extreme deer pressure, which occurs during periods of environmental stress like severe drought or heavy winter snow. When preferred food sources are scarce, their foraging selectivity decreases drastically, and they will consume nearly any available vegetation, including plants they would otherwise ignore. In such cases, the plant’s bitter chemical deterrents are outweighed by the animal’s desperate need for survival and caloric intake.
Browsing damage is sometimes observed on young or newly established Bidens plants before their chemical defenses are fully mature. Immature growth may not have accumulated the full concentration of defensive polyacetylenes and flavonoids found in established foliage. The tender, new shoots are occasionally sampled by deer testing the palatability of the garden, which results in damage during the establishment phase. This initial browsing can lead the gardener to believe the plant is not resistant.
The intensive breeding of modern ornamental cultivars for unique traits can also affect their resistance level. While most Bidens ferulifolia types are highly resistant, some heavily hybridized varieties focusing on novel flower color or form may have slightly diluted concentrations of bitter compounds compared to wild ancestors. These less-bitter varieties may be sampled more frequently, especially when deer pressure is moderate. These external conditions are the usual reasons why a resistant plant is occasionally eaten.
Practical Strategies for Protecting Bidens
When a plant’s natural defenses are overcome by high browsing pressure, gardeners can employ several strategies to mitigate damage. Physical exclusion is the most dependable method, as it completely eliminates deer access to the plant material. Installing temporary barriers, such as fine netting or wire cages, around new plantings until they are well-established is a highly effective tactic. For permanent protection, a fence height of at least six to eight feet is recommended to prevent deer from jumping over the barrier.
Chemical repellents offer a secondary layer of protection by making the plants more distasteful or alarming to the animals. Repellents containing ingredients like putrescent egg solids or capsaicin are taste-based. They work by coating the foliage with an offensive flavor, reinforcing the plant’s inherent bitterness. Scent-based deterrents, which often use strong odors like blood meal or essential oils, function by signaling to the deer that the area is an undesirable foraging location.
For maximum effectiveness, repellents must be applied consistently and rotated between different formulas to prevent deer from becoming habituated to a single deterrent. Reapplication is important after significant rainfall or when new, unprotected growth emerges. Gardeners can also strategically place Bidens close to areas of frequent human activity or interplant them among highly repellent species, such as lavender or aromatic herbs. This creates a deterrent zone that leverages the deer’s natural caution.