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Calendula

1. Soil and pH Needs

In the landscape, Calendula prefers rich, well drained soil, but will grow in average to slightly poor soils. 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 5.5 – 7.0 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. 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. 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.  A 2 - 3 inch layer of mulch spread on the soil 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

Calendulas are light feeders in the landscape; and prefer cool weather.  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. Do not over fertilize. Too much nitrogen will promote excessive foliage growth at the expense of flower production. 

 

3. Disease

Calendulas are prone to powdery mildew in the landscape. Powdery mildew is the result of overhead irrigation, improper plant spacing, poor air circulation, or watering too late in the day. It can be avoided by employing proper cultural practices. A plant with powdery mildew will have white fungal growth on the upper surface of the leaves and it may also form on the flowers. 

 

 

Plant heavily infected with powdery mildew

 

4. Insects

Aphids and whiteflies are insects that commonly attack Calendulas.  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. 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.  

 

 

Aphid infestation

 

Adult whiteflies

 

5. Tips for Success

  • Calendula will thrive in full sun, and a well-drained soil.
  • Calendulas are deer resistant. 
  • Remove weeds from the plantings that compete with Calendulas for moisture, nutrients and light.