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Series: Magellan™, Short Stuff™, Swizzle™, Uproar™, Zowie!™


1. Soil and pH Needs

In the landscape, zinnias are tolerant of all but wet, poorly aerated soils that can cause root rot. 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 Canadian sphagnum peat moss and well-rotted compost are good sources of organic matter that will aid in aeration. If possible, incorporate two to three inches of organic matter into the top six inches of soil. 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 growth; 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. Zinnias can cope with mild drought, but they appreciate regular watering. When watering, try to soak the soil thoroughly, but do not water so freely that puddles collect on the ground. Avoid getting moisture on the leaves which can cause mildew. Water early in the day so foliage that gets wet can dry out before evening. In the summer when rainfall is sparse, try to provide about an inch of water a week.  A 2 or 3 inch layer of mulch spread on the soil over zinnia roots discourages weeds. This mulch also helps prevent evaporation of moisture from the soil and it harbors beneficial organisms to help protect plants from pest problems. The mulch will decompose over the summer, adding valuable organic matter and some nutrients to the soil. 


2. Fertility

Zinnias aren’t heavy feeders in the landscape; and can have a crop life through the summer until first frost.  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 applied 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. Do not over fertilizer. Too much nitrogen will promote excessive foliage growth at the expense of flower production. High nitrogen levels will also cause soft, weak stems, and excess boron will slow flowering. A boron deficiency can cause abortion of terminal flower buds and reduced branching. 


3. Disease

Zinnias are prone to several diseases in the landscape. Problems such as alternaria leaf spot, bacterial leaf spot, and powdery mildew commonly are the result of overhead irrigation, improper plant spacing, poor air circulation, and watering too late in the day. These diseases can be avoided by employing proper cultural practices. Alternaria leaf spot will result in large reddish brown or purple spots, at first being round to oblong but later becoming irregular in shape and having gray or tan centers that may drop out leaving a hole. Bacterial leaf spot can be identified by the presence of reddish brown to dark brown spots that are angular and with a prominent yellow halo. A plant with powdery mildew will have white fungal growth on the upper surface of the leaves and may also form on the flower petals. 


Symptoms of bacterial leaf spot (spots with yellow halo) and alternaria leaf spot (purple spot without halo)


4. Insects

Aphids, thrips, and whiteflies are insects that commonly attack zinnias.  Aphids feed on the plant sap. This halts growth, causing the leaves to become deformed. Aphids also excrete honeydew which causes sooty mold to develop and makes the plant unsightly. Thrips cause damage to the plant by piercing and sucking cells on the leaf surface. This causes silver-grey spots to form on the foliage. At high infection rates, the leaves may wither. Both the adult and larval stages of whiteflies extract food from the plant. This affects the plant growth resulting in growth reduction. Remember that healthy, vigorous plants are less susceptible to pest damage than unhealthy plants.  


Adult aphids


Adult Western Flower Thrips


5. Tips for Success

  • Zinnias prefer full sun, and a well-drained soil.
  • The zinnia is hardy and drought-tolerant; it needs minimal, but deep water, once established. 
  • Pinch off spent zinnia flowers (dead head) to encourage continuous blooming and maintain a neat appearance. 
  • Remove weeds from the plantings that compete with snapdragons for moisture, nutrients and light.

Information provided by Syngenta Flowers