Answer: Verbenas can suffer from a variety of problems, but most occur when they are grown in low light, poorly drained soil, or when the soil stays excessively moist from excessive watering. Poor air circulation from over crowded conditions can also lead to disease problems.
Powdery mildew appears as a white powder fungus on the surfaces of leaves. It most often infects verbena that does not receive enough sunlight, or is under stress from severe drought or other causes.
Botrytis blight often occurs under overly moist conditions. Flowers turn brown and sometimes a gray, fuzzy fungus is visible.
Root rot cause by Pythium or Rhizoctonia fungus may occur in overly moist soil.
Verbenas require a location that receives full sun throughout the day. They must have well-drained soil. They will not tolerate overcrowding with poor air circulation, shade or soil that stays overly moist. Most problems of verbenas occur in improper growing conditions.
Verbenas are best planted in the spring or summer in the upstate and piedmont regions of South Carolina. They may also be planted in the fall in the Coastal region. Pinch the tips of the branches at planting time to encourage dense branching and a fuller plant.
Newly planted verbenas will need to be kept moist for the first few weeks until the roots have spread into the surrounding soil.
While established verbenas are drought tolerant, performance, bloom, and growth rate will be reduced if they are too dry for a long period. During their blooming period, give them a thorough watering once a week if they do not receive an inch of rain that week. Avoid overhead watering.
If bloom slows during the summer, trim the whole plant back by about one fourth of its height and spread, water thoroughly and fertilize lightly. The plant will return to bloom within 2 to 3 weeks.
A light application of a complete fertilizer such as 16-4-8 in mid to late spring and again after trimming back will revitalize plants, but additional fertilization is not generally required. Plants growing in very sandy, poor soil may need more frequent fertilization.
In the fall you can trim back verbenas lightly to give a neater appearance to the garden, but do not cut severely until spring as new growth begins to appear. Overly severe fall pruning can reduce cold hardiness and plants may not survive a cold winter.
Most verbenas are short- lived, so you should plan on replacing them after two or three years. However, some species can re-seed and naturalize in the garden.
Verbenas, especially the trailing and moss types, grow very well in containers. Fertilize container grown plants either with a controlled release fertilizer, or with a liquid fertilizer once a month. Container grown plants should be watered more frequently, and not allowed to dry out.
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