Butterflies love verbena flowers so prepare yourself for the onslaught when growing the plants. Generally trouble-free, verbena loves the heat and blooms from summer until late fall. Occasionally verbena has problems, either from pests, diseases or the way in which it is cared for. Sometimes the solution to the problem lies in choosing a type of verbena that is better adapted to your region -- annual verbenas don't do very well in hot, humid areas, for instance. Consider as well that although the verbena may appear to be dying, it may be exhibiting stress because of a problem you might be able to remedy.
A number of insect pests that may infest the verbena, but most of them won't kill the plant. Spider mites, on the other hand, are capable of defoliating and killing the verbena. Common in hot, dry regions, spider mites tend to attack neglected plants, sucking the juices out of the foliage. Foliage turns yellow or brown and drops from the plant. It may appear to be dying and, if the infestation is heavy, the plant may, indeed die. Look for webbing on the foliage and stems and take action at the first sign of an infestation. Use a miticide, listed for use on spider mites, according to label instructions.
Verbena is generally disease resistant except for one fungal disease -- powdery mildew. Although the disease probably won't kill the plant, it may appear that it is dying. This disease takes hold when its spores germinate on moist foliage. Then, if the weather is warm and dry, it thrives. Turn over any foliage that turns yellow and look for a white, powdery patch. If you find these, cut off infected portions of the plant and treat the rest immediately with a fungicide, according to label instructions. Prevent powdery mildew infections by growing the verbena in an area of the garden that receives eight to 10 hours of sun per day.
Verbena flowers again after the first flush if one-fourth of the top growth is cut back. If left unpruned, it may appear to be dying as it goes to seed. Overwatering is deadly for the verbena, so water only when the soil is dry. Improper fertilization can also have a deleterious effect on verbena. Too much fertilizer or fertilizer applied on a hot day can burn the plant's roots, killing it. Always apply at the rate suggested on the label for the size of your plant, apply it on a day when temperatures will remain below 80 degrees Fahrenheit and water deeply after the application.
Because of verbena's intolerance of excessive moisture, check the drainage of the soil where it is planted. If water puddles on the surface of the soil for more than an hour after it rains, you may want to consider transplanting the verbena into an area with better drainage. A good way to test the drainage in other parts of the garden is by digging a 4-inch deep hole and inserting an empty 46 oz. can -- with the top and bottom removed -- into it. Fill the can with water and time how long it takes the water to drain. If it takes longer than one hour to recede 2 inches, either amend the soil or choose another location.