A discussion of water sources for bees would not be complete without a mention of guttation. Drops of liquid that you see along the edges of a leaf—or sometimes at the very tip—are actually drops of xylem sap. The process that delivered the sap to the leaf margins is called guttation.
Drops of xylem sap
The roots of plants take up water from the soil. While they soak up the water, they also take up salts, minerals, fertilizer, and pesticides. As the osmotic pressure in the roots increases, it pushes the sap upwards throughout the plant, preventing the plant from wilting during the night.
The excess sap is exuded through pores and can be seen as little droplets.The pores where the xylem sap oozes out are called hydathodes. Unlike leaf stomata, which are open only during the day, hydathodes are open all day and all night. Because the xylem sap contains many different materials, deposits of dry matter are often left behind when the water evaporates from the leaf.
Not all plants produce guttation drops. The phenomenon is restricted to herbaceous plants and some vines. Trees and other woody plants are too big for the osmotic root pressure to push liquid throughout the plant.
Not transpiration, not dew
Guttation should not be confused with transpiration or dew. During the day, a plant loses moisture through the stomata in a process called transpiration. Transpiration, which occurs in all terrestrial plants, helps to keep the plant cool, whereas guttation does not. And unlike guttation, the water from transpiration escapes in the form of vapor—not liquid—and it is pure.
Guttation droplets are easily distinguished from dew by their location. Guttation occurs only at the hydathodes at leaf tips or margins, but dew condenses anywhere on the surface of a leaf. Guttation drops form a regular pattern, whereas dewdrops do not.
Guttation and bees
On dry days, bees of many species can be seen drinking guttation droplets. Unfortunately, some research has shown that bees drinking the xylem sap of plants treated with systemic insecticides can be poisoned by the liquid. Guttation occurs in many grass-family plants, including maize and other grains that are commonly treated with systemic insecticides.
This morning, during a quick inspection, I found a number of plants with guttation drops in my garden, including grape, lemon balm, lamb’s ear, tomato, and nasturtium. My bees seem to like the lamb’s ear the best, and I often see them crawling along the leaf margins, lapping up the minerally (and pesticide-free) water.