bee forage

Xylem sap for honey bees


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.



Guttation drops along the perimeter of a lamb’s ear leaf. © Rusty Burlew.


Guttation oozing from a grape leaf. © Rusty Burlew.


This morning, all the lamb’s ear leaves were edged with pearls. © Rusty Burlew.


Leaves of lemon balm decorated with guttation drops. © Rusty Burlew.


  • Now, THAT was something worth learning about today. There is a collection pool next to my hives that always has water in it and a few frogs. I don’t worry about water throughout the year, but it certainly was interesting to find out about guttation. Will be looking out for it early in the morning.

  • Splendid pictures, Rusty!
    And thanks for the reminder about systemic pesticides.
    Someone asked this week if we could apply anything to the roots of Ash trees to kill the Emerald Ash Borer which is devastating our woods. I explained that it might kill the borer larvae, but it would also harm any insect that foraged the Ash blossoms. The answer: “Ash trees have flowers?”
    Honestly, where do people think little trees come from? No wonder we have allowed chemicals to do so much harm, if this is the state of science understanding.
    Thanks again!
    Shady Grove Farm
    Northern Kentucky

  • So this is a different phenomenon to extra-floral nectaries, as it sounds like the xylem sap doesn’t contain sugar?

    • Emily,

      Right. Extra-floral nectaries actually secrete nectar, but the hydathodes are part of the water transport system.

  • I learned something new today! I’ve never heard the term “guttation” before — and here I thought I knew my plants pretty well. That was fascinating, and your photos are stunning as always. Thanks Rusty for always sharing your knowledge so freely.

  • What an informative article. Very interesting and the photos are stunning! You have taught me a lot about bees, plants and our natural world. I mention your blog at our beekeeping club, to local gardeners, regular folks, and especially people who don’t seem interested in bugs, plants, birds or much of anything that isn’t connected to electricity. Your blog opens a new world to a lot of people. Thanks a million!

  • Thank you! I have seen this on my squash plants, and completely misunderstood. I love learning new things about my garden. Awesome photos as well. Made my evening 🙂

  • I, too, learned new vocabulary today. Very well explained. I’ll be on the lookout. Photos are awesome!

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