Nutrient deficiencies of perennials: Part 4 of 12

Clump verbena (Verbena canadensis)

Clump verbena (Verbena canadensis), also called rose verbena or rose vervain, is a perennial native to North America and a member of the eponymous Verbenaceae family. Verbena canadensis is also a parent for many of the commercially available vegetatively propagated Verbena x hybrida cultivars.

The popular and widely grown ‘Homestead Purple’ was discovered by University of Georgia horticulture professors Allan Armitage and Michael Dirr. Most likely a V. canadensis cultivar, it’s usually sold simply as Verbena ‘Homestead Purple’ (no specific epithet -- sort of like Cher ). Hardy to USDA Hardiness Zone 7 (and sometimes Zone 6), the plant is usually covered in butterfly-attracting clusters of purple flowers. Its long flower time rivals that of most annuals. Its mounding habit also also works well in hanging baskets and containers.

‘Homestead Purple’ is easy to propagate and quick to fill a quart pot (seven to nine weeks) and has relatively few pest problems.

Since verbena grows quickly in the greenhouse, deficiency symptoms can appear rapidly, affecting plant size, quality and time to flower. In our study, calcium- and iron-deficient plants displayed visible symptoms within seven days of the start of the study. As some elemental deficiencies have similar visible symptoms, tissue (leaf) analysis by a reliable laboratory is the best way to confirm your initial diagnosis.

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Researchers at Virginia Tech University induced and documented deficiency symptoms in vegetatively propagated ‘Homestead Purple’ verbena and compared them to controls grown with a complete nutrient solution. We used a hydroponics system to completely exclude the element of interest; this also afforded a great view of root system development.

Pictures related to the nutrient deficiencies series may accessed by viewing the PDF files of the pages that originally appeared in GMPRO magazine: Page 1. Page 2. Page 3. Page 4. Page 5. Page 6.

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Nitrogen (N)

Initial nitrogen deficiency symptoms aren’t visible for two weeks, and then the leaf margins and stems take on a purplish cast.

As nitrogen deficiency progresses, younger leaves began to cup and stems cease elongating, resulting in shorter internodes compared to the control plants. Previously purple leaf margins are necrotic.

Finally, severe chlorosis of the lower (mature) leaves occurs. Nitrogen’s influence on the shoot-root ratio is demonstrated here. The plants grown in the solution without nitrogen (-N) have greater root branching and mass compared to the control, though shoot mass is much less.

Phosphorus (P)

Mature foliage has gradual darkening, then marginal necrosis. Flowering is delayed and overall plant size is much smaller than the control. The entire plant is dark, dull green with purple-pigmented stems.

Note the phosphorus-deficient plant exhibits excessive root growth relative to shoot mass after five weeks for the phosphorus-deficient plants. This may not occur in the growing medium, but demonstrates what happens when root growth is not restricted by growing medium and/or container.

Potassium (K)

Potassium deficiency symptoms first appear as interveinal chlorosis, most noticeable on recently mature leaves.

Though days to flower is the same, and plant shoot and root mass remain similar to the control, the foliage of the potassium-deficient plant looks bad. Leaf margins and chlorotic flecks turn a papery tan.

Calcium (Ca)

Calcium deficiency symptoms appear rapidly. Within seven days, light interveinal chlorosis plus necrotic spots mark the young and recently mature leaves. The newest foliage cups inward.

Calcium-deficient plants decline rapidly. Shoot growth is arrested, new foliage is distorted and the margins on mature leaves become necrotic.

The importance of calcium in root formation is evident as the deficient root system is very stunted with a fishbone appearance. New root tips are brown.

Magnesium (Mg)

Initially, the verbena grown without magnesium showed lime-green interveinal chlorosis on younger, recently mature leaves. Most symptom diagnosis references state magnesium deficiency is first visible on mature or lower leaves.

Interveinal chlorosis spreads to all the foliage of magnesium-deficient plants, with mature leaves showing necrosis along margins in advanced stages 

Sulfur (S)

Sulfur-deficiency symptoms are slow to develop on the foliage and are not visible until three weeks into the study. However, growth is stunted and there is some interveinal chlorosis of recently mature leaves.

As symptoms progress, shoot growth of the sulfur-deficient verbena is arrested, flowering delayed and the entire plant takes on a chlorotic, yellow-green cast.

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Boron (B)

Deficiency symptoms for boron first appear in the youngest tissue. New leaves are thick and wrinkled.

As boron deficiency advances, young and recently mature foliage is chlorotic and distorted while the oldest leaves look normal.

Like calcium, sufficient boron is crucial in the formation of new tissue (note the brown and stunted root tips).

Copper (Cu)

Copper deficiency will slow or halt internodal elongation, hence the rosetting here in early stages.

As copper-deficiency symptoms progress, mature leaves have necrotic spots and margins, younger leaves have interveinal chlorosis, and overall shoot and root growth are stunted compared to the control.

Iron (Fe)

There was a rapid onset of iron-deficiency symptoms. Within four days of the start of the study, the youngest verbena leaves are already chlorotic.

Within seven days, necrotic spots appear on young foliage. The chlorosis progresses from leaf base to tip over time.

In less than two weeks, young foliage is curling and severely chlorotic with necrotic spots and margins.

Manganese (Mn)

Manganese-deficient verbena is slow to develop visual symptoms. But when chlorotic spots finally appear on mature leaves, they quickly become necrotic.

Six weeks into the study, the newest leaves are bleached and chlorotic, while mature leaves are very dark green with flecks of necrosis. 

Zinc (Zn)

After two weeks in the zinc-deficient solution, older leaves display necrotic spots.

Advanced zinc deficiency symptoms include folding along the midvein of young, slightly chlorotic leaves.

- Holly L. Scoggins, Allison Byrd and Velva A. Groover

Holly Scoggins is associate professor, Allison Byrd is a former graduate research assistant and Velva Groover is a senior research technician, Virginia Tech University, Department of Horticulture, Blacksburg, VA 24061; (540) 231-5783; perennials@vt.edu.

The authors thank Fred C. Gloeckner Foundation for grant support, Yoder-Greenleaf for plant material and Quality Analytical Laboratories for tissue analysis. The authors thank Virginia Tech University metabolomics specialist Joel Shuman for technical assistance, and partners in the study, University of Florida assistant professor James Gibson and his staff and students at West Florida Research and Education Center in Milton , Fla.

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