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Verbena

Series: Obsession™, Tuscany®

 

1. Soil and pH Needs

Verbenas need a soil that drains well, but not one that is particularly rich or fertile. Flowering will suffer if soils are too fertile. However, Verbena does not perform well in wet, poorly drained soils or in cool spring weather. Heavy clay soils will retard root growth and starve the plant of oxygen, which can lead to root diseases. Soil should be amended with fully composted organic matter prior to planting and in the case of clay soils the addition of pea gravel equal to at least 25% of the soil volume will aid in aeration. If adding organic matter, be aware that organic matter that has not been fully composted will tie up nitrogen and sulfur leading to nutrient deficiencies and poor growth.  Soil pH should be 5.8 - 6.2 for optimum uptake of iron; and should be tested prior to planting but after adding any amendments. This can be done quite easily with a portable pH tester available for about $100. Mix one part soil with two parts distilled water, stir well and wait thirty minutes, then follow the instructions for using the pH meter. If pH is too low, the addition of lime is warranted. However, the rate will vary depending on how much you need to raise the pH and the type of soil you are dealing with. Clay soils or those with a lot of organic material have a high buffer capacity and require more lime than sandy soils, which have a low buffering capacity. A good soil testing lab can determine the lime requirement index (LRI) of your particular soil and can recommend types and rates of lime to use. The frequency of watering will depend on the type of soil, weather conditions and the amount of mulch. Mulch will not only reduce soil water evaporation but will also reduce splashing of water onto the lower leaves, moderate soil temperatures and reduce weed competition. Because they're quite drought-tolerant, verbenas seldom need daily watering.

 

2. Fertility

Verbenas are low to moderate feeders in the landscape; and can have a crop life of four – six months. An application of a balanced controlled-release fertilizer (CRF) such as 10-10-10 or 14-14-14 with a 3-4 month formulation applied to the bed at planting will keep the plants well fed, depending on the amount of irrigation required and the average daily temperature. In areas of high irrigation and high temperatures it may be better to use a formulation with a slower release rate since higher temperatures will cause the fertilizer to be released quickly. Fertilizers that are not CRF are not recommended since the fertilizer will be leached out of the root zone long before the crop needs the nutrients. In high pH soils (alkaline soils) additional iron or iron sulfate may be beneficial to reduce chlorosis (yellowing) of the foliage.  Yellow foliage on verbena may also be caused by a nitrogen deficiency within the plant. Both can be corrected with the proper supplemental fertilizers. 

 

3. Disease

Water correctly to prevent most of the fungal diseases that can cause problems for verbena such as: botrytis and powdery mildew. Botrytis can be identified as tan spots will develop on the leaf surface. Established plants will show rotting in the crown. Powdery mildew will appear as white fungal growth on the upper leaf surface and may also form on the flowers. The best preventative is to use drip irrigation. At the very least, direct water from the hose onto the soil or mulch, and not on the plants themselves. 

  

Symptoms of Botrytis on verbena

 

Powdery mildew on the upper leaf surface

 

4. Insects

You may occasionally find aphids, thrips or spider mites on the verbenas. Thrips cause damage to the plants by piercing or sucking out the cells on the leaf surface. This causes silvery grey spots on the foliage. At high infection levels leaves may wither. Thrips are also a primary vector for virus transmission. Aphid nymphs and adults feed on the plant sap by piercing the tissue. This slows growth and can cause leaf curling. Aphids also excrete honeydew. The honeydew will lead to sooty mold formation which will reduce growth and production. Spider mites feed on the underside of the leaves. This feeding causes yellow spots to form which results in decreased plant growth and production. Remember that healthy, vigorous plants are less susceptible to pest damage than unhealthy plants.  

 

Adult aphids

 

Thrips damage to the flowers, white flecking on the petals

 

Signs of spider mite damage

 

5. Tips for Success

  • Verbenas possess excellent heat and drought tolerance.
  • Cool temperatures, high fertilization, and overwatering are the most common problems in the landscape.
  • Deadheading the faded blooms will encourage re-blooming.
  • Verbenas need 8-10 hours of direct sun and a well-drained soil.
  • Remove weeds from the plantings that compete with verbena for moisture, nutrients and light.

Information provided by Syngenta Flowers