New York Times By David Taft 7/5/13
The flowers of the prickly pear cactus (Opuntia humifusa) are evolution’s answer to a bee’s dreams. They are filled with pollen, abundant in season, and easy to locate. For humans, they are also stunningly beautiful, if ephemeral. Each satiny yellow bloom lasts about a day, opening broadly in the first hours of the morning when temperatures are still relatively cool, and closing again as the sun sets. A good-sized plant displays up to a dozen two-inch flowers every day from mid-June through early July in and around New York City.
The cactus’s flower is shaped like an open bowl, with dozens of pollen-tipped stamens. Both native bumble bees and introduced honey bees find the blossoms irresistible, and are a constant presence from the moment they open. Patience pays when observing the arrangement between these partners. Finding an open flower, a bee dives in, swimming through masses of pollen-laden anthers and vibrating its body rapidly to loosen the pollen grains. It turns out this activity is hardly necessary: prickly pear cactus stamens are “thigmotactic,” that is, they are mobile, and bend inward when stimulated. The stamens proactively bend in toward the bee, dusting its fuzzy coat with pollen. This cactus pollen is eventually combed off by the bee and packaged for transport to the hive.
You can try this yourself; carefully brush a pencil against the stamens of a newly opened prickly pear flower. The response is quite remarkable.
Growing in full sun in well-drained soils, the plants can be found in bone-dry conditions, in sandy, rocky and otherwise poor soils, but they are very adaptable. I have encountered them thriving in areas where the water table is right at the soil’s surface. The plant’s paddle-shaped pads are unmistakable, and since there are no other cactuses native to the New York area, identification is easy. A cactus seems like an anomaly this side of the Mississippi, but the eastern prickly pear can be found from Florida north to Massachusetts, with outliers in Texas and Montana.
In the fall, as the weather cools, the cactus pads (actually modified stems) desiccate, and become rumpled, greenish-purple strips that lie flush against the ground. This behavior prevents water from freezing and expanding in the stems, ultimately killing the plant.
Lying prostrate also allows even small amounts of snow to cover the dormant cactus. Snow is a cold but efficient insulator, and temperatures under the snow hover at just about freezing, never far below. The same pads rehydrate and spring back vertically, only to sprout flower buds and new stems as the weather warms.
There is no discussing a cactus without at least some mention of spines. Merely modified leaves, the spines of the native prickly pear are accompanied by glochids, which are fine and hairlike. Don’t let their size fool you; there are few things as irritating as these almost-invisible spines lodged in the joints of fingers or other body parts.