Guttation adorns even common weeds with early morning jewels

Before there were gardens, there was Dr. Bill Lenoir, my undergraduate botany professor. His classes kindled a lifelong passion for plants. He inspired desire to work beyond the syllabus, to explore questions that began in the lecture hall and ended with late night hours in the laboratory. I think of him often, recalling lectures and labs that deepen my understanding of observations made on garden walks.

If memory serves – it’s been 40 years – Dr. Lenoir had us grow tomato plants to learn about guttation, the production of small droplets of xylem sap at the leaf margins of some plants. Not to be confused with dew, which covers the entire leaf, guttation fluid appears precisely where leaf vein tips meet the leaf edge, forming a ring of droplets that describes the outline of the leaf.

Guttation occurs when soil moisture levels are high and loss of water by transpiration through open stomates (pores in the leaf) is low. Since stomates of most plants close in the dark, we find the evidence of nocturnal guttation in the early morning light.

During the night, uptake and accumulation of salts by root cells leads to osmotic uptake of water into those cells. The increase in water leads to root pressures that can be relieved only by forcing water out of specialized cells at the leaf margin, cells called hydathodes. The root pressures are minimal, soon equalized by atmospheric pressure, and thus the amount of water forced out of the leaf is minimal – just enough to adorn the early morning leaf with sparkling jewels.

Guttation droplets are ephemeral jewels, evaporating as the sun warms the leaf. A thin residue of dried salt on the leaf margin provides the only evidence that they were ever there.

The recent spell of cool, wet weather was perfect for guttation mornings. Early one sunny morning after several days of rain, I wandered slowly around our garden, tripod-mounted camera in hand, a sense that Dr. Lenoir was at my side. I photographed guttating leaves of radish, strawberry, sunflower and, of course, tomato plants. Potted nasturtiums were guttating, a tiny, perfectly round drop of fluid expelled from the end of each conspicuous leaf vein.

Garden weeds also reflected sunlight from sparkling drops of sap. The most beautiful jeweled leaves were those of common mullein (Verbascum thapsus), a Eurasian weed recognized by its large flannel-covered leaves, a characteristic that accounts for several other common names, including blanket-leaf, feltwort and flannel-leaf.

Mullein’s frost-green leaf is heavily veined, particularly when viewed from below, and guttation droplets form not only at vein tips along the margin but also from major veins within the leaf blade.

Low-angled light gave the guttation droplets on mullein the appearance of faceted gemstones. And so, on this particular morning, a common weed became the most impressive plant in the garden.

Mullein is a biennial, growing vegetatively the first year, flowering the second summer before dying in the fall. Unable to compete successfully with cultivated plants, only three mullein plants growing along the edge of a strawberry bed will flower this year in Marjorie’s garden.

While each knee-high plant will produce up to 150,000 seeds, few will germinate and even fewer plants will mature to flower in their second year. This weed is not likely to become a serious problem. It is well worth keeping a few around just to see their jeweled leaves sparkle in the early morning of a summer day.