Question: I have mold on my zinnias. How can I treat this problem and prevent it from happening next time?
D.V., Corona del Mar
Answer: The zinnia's ability to tolerate summer heat, provide continuous blooms and require less water than many other annuals makes it a prime choice for the summer garden in Southern California.
You have just discovered its main drawback, however: powdery mildew.
This white coating on the leaves causes the foliage to shrivel and die and can also affect the flowers, shortening their length of bloom and making them unsuitable as cut flowers. Powdery mildew is more prevalent in foggy areas (such as along the coast), when the plants are overhead-watered, in areas where dew lingers, and in late summer-early autumn when days are warm and nights are cool.
Most of the large-leafed, large flowering zinnias are prone to developing mildew. Good cultural care and variety selection can help minimize this problem.
Cultural requirements for zinnias:
* Select an area that receives full sun and has good drainage. Before planting, add a generous amount of organic amendments and a general all-purpose fertilizer.
* Sow seeds where you want them to grow, or set out nursery plants once the weather has warmed up (May through July).
Planting via seed often produces sturdier plants. Zinnia seeds require light to germinate, so only cover the seed with a thin layer of soil.
Try to mist the area two times a day until the seeds sprout. Once this occurs and they are firmly rooted, stop misting and start watering them at ground level so as not to wet the foliage.
Once the seedlings are 3 to 4 inches tall, switch from frequent watering to occasional deep soaking. (Apply about 1 inch of water per week.)
* Provide monthly light feedings throughout the summer.
* To encourage branching, pinch above the third set of leaves. Remove spent flowers to encourage increased blooms.
Tips to minimize occurrence of powdery mildew:
* Allow adequate spacing between plants (increased air circulation will decrease mildew and other fungal diseases).
* Water early in the day and avoid getting water on the foliage.
* Plant in an area with adequate sun.
* Where infection is limited, try to prune out and discard diseased tissue as soon as it appears.
* If you decide to apply a fungicide, consult a California Certified Nursery Professional at your local nursery or farm supply.
* Choose varieties that are resistant to powdery mildew. Large double-flowered varieties are likely to develop mildew, but they will usually bloom for several months before the mildew overtakes them.
There are three species of annual zinnias:
* Zinnia elegans. The colors of these popular flowers range from deep red, to violet, to snow white, and come in many strains, from dwarf plants with small blooms to large plants with 5-inch blossoms.
Powdery mildew tends to strike most in this species if late-summer nights are humid. It tends to only affect the leaves, not the stems or blooms.
* Zinnia angustifolia (Z. linearis). These are more compact plants with open-faced single blooms in red, orange, yellow and white.
They do not require removal of spent blooms. Within this species you will find the Pinwheel series. They are a hybrid between Z. angustifolia and Z. elegans and are touted as being mildew resistant. Their flowers are 3 inches in diameter and come in shades of cherry, orange, salmon, white and rose. Other varieties of this species that have shown some promise of mildew resistance are members of the Star series such as 'Orange Star' and 'Star White.'
* Zinnia haageana. This species has compact plants with narrower leaves than on common zinnias. The variety, 'Old Mexico' has shown mildew resistance.
Have a problem in your yard? University of California Cooperative Extension (UCCE) Master Gardeners are here to help. These trained and certified horticultural volunteers are dedicated to extending research-based, scientifically accurate information to the public about home horticulture and pest management. They are involved with a variety of outreach programs, including the UCCE Master Garden hotline, which provides answers to specific questions. You can reach the hotline at (714) 708-1646 or send e-mail to ucmastergardeners @yahoo.com. Calls and e-mail are picked up daily and are generally returned within two to three days.