Q. I am trying to grow annual vinca in zone 5. I got some this year and planted in early June. They are in a location that faces west and gets sun all afternoon. Right after planting, we had a lot of rain. Now they have all turned yellow for some reason. Is it the result of an iron deficiency? Or the pH level? I think the pH level is 7. I didn’t use any fertilizer after planting, but I applied Iron-Tone a few days ago with no results. —Maggie Li, Chicago
A. I lived briefly in the Chicago area many years ago. I can imagine a perfect storm of conditions, early in the season, that could make it difficult to start and grow annual vinca there. Although your problems may be pH-related, there may be several other factors at play.
Catharanthus roseus, a tropical perennial, is used as an annual in our landscapes. Originally from Madagascar, it is often called Madagascar periwinkle. This is an excellent plant – in the right place. Unfortunately, this can be quite temperamental. The annual vinca prefers warm to hot weather, tolerates drought and likes good, well-drained soil. If planted too early, before the warm weather arrives, it will languish. Too much water will result in plants with yellow to pale green foliage, stunting and eventual death. And to your question, it requires a pH of 5.4 to 5.8. Outside of this range, it can be prone to several fungal pathogens and, as you noted, iron deficiency. And finally, it is sensitive to high levels of fertilizer.
Again, Vinca likes it hot and dry, which is not exactly what you might expect in your area in late spring. With a combination of cool weather and rain, your plants have probably had a hard time getting established. As conditions improve, they can get out. But you seem to have focused on pH as the issue, and it certainly could be. It is one of many plants (see azalea) that suffer from iron deficiency associated with low pH. Interveinal chlorosis, which first appears on older leaves and progresses, is a classic symptom of iron deficiency.
The product you used, depending on how you applied it and the severity of the problem, may offer some relief, but it’s not immediate or long-lasting. It may take more than a few days. I suggest you try a foliar spray using a chelated iron product. This form of iron is and remains soluble in water. It will only apply to the sprayed leaves: it is not easily transported to other parts of the plant, including new growth. While this should result in a faster response, it’s also short-term and temporary, and you may need to spray again.
To get it right and get a more permanent solution, you need to address the underlying cause of the problem: low soil pH.
First, it requires a bona fide soil test to accurately determine pH, as well as other nutrient levels. I recommend checking with Illinois Extension for test kits and instructions. (An extension author’s account is here; he paid $15 for a kit: tinyurl.com/ExtSoilpH.)
There are two approaches to dealing with low soil pH:
Add acidifying agents to the soil. This method assumes that the soil has enough available iron (as verified by a lab test). According to Oklahoma State University extension recommendations, elemental sulfur can be incorporated into the top 3 to 6 inches of soil at a rate of 2 to 4 pounds per 100 square feet. Alternatively, aluminum sulfate can be applied to the surface at the rate of 12 to 18 pounds per 100 square feet, then watered in. Excessive application can lead to aluminum toxicity. Either of these methods will take two to four years to achieve the desired correction, but they should last several years.
Apply fertilizers containing iron. Use this approach when the soil lacks available iron (as verified by a lab test). Iron sulfate can be applied in granular form, worked 3 to 6 inches into the ground at a rate of 10 to 14 pounds per 100 square feet. The correction time will be two to four years, but the treatment should last several years. Alternatively, iron sulfate can be mixed with water at the rate of 2.5 ounces per gallon and applied to foliage or soil surface. The correction time will be quick, but the effects will last less than a year. To avoid damage to foliage, any overspray of these materials should be done early or late in the day, or on cloudy days – not on sunny days.
Again, for your vinca, I would consider the quicker, short-term approach this season.
Looking ahead, you should think about what plants you would like to grow in that particular location. I’m not suggesting you give up on Vinca, but plan ahead to accommodate its somewhat demanding nature. Don’t forget the soil test to give you the information you need.
And one more thing
I’m curious how everyone’s gardens are doing this season. And on another note, please share the recycling tips and methods you use in and around your garden. Email me and we’ll compare and share your ideas.
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