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Description: Armstrong Growers is a wholesale grower of quality plant products and programs, serving independent garden centers, commercial landscapers, maintenance companies, hotels, and resorts in California, Arizona, Nevada and Southern Utah.
Vinca needs a soil that drains well, but not one that is particularly rich or fertile. Flowering will suffer if soils are too fertile. However, Vinca 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 between 5.5 - 6.2 for optimum uptake of iron and suppression of Thielaviopsis; 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, vinca seldom need daily watering other than what they receive with rain; in prolonged periods of drought, however, watch that the soil doesn't get too dry.
Water correctly to prevent most of the fungal diseases that can cause problems for vinca such as: root rot, botrytis, alternaria leaf spot, and aerial phytophthora. Of these, aerial phytophthora is the most prevalent; where overhead watering splashes soil and fungus spores onto lower foliage and stems. Lesions develop on leaves and stems, killing the shoot but leaving a healthy root system. 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.