Purple/black leaves fades to ugly

(Question)

 

I grow purple potato plants every year and every year I have the same problem. When I first get them home from the nursery in spring, I grow them inside to protect them from the squirrels until the acorns ripen. When I put them outside they go from purple black and fade to ugly. (see picture) Then I cut them back to get the pretty color back but it fades to ugly again. I water when the soil is dry all the way through and then give it a long drink until it runs through.IMG_4160

• North Central Texas
• Zone 9
• 100 to 90 Degrees
• Full Sun 4 6 Hours a Day
• Hanging Plastic Pot
• Good Drainage
• 3rd Floor Apartment Balcony

I wonder if it is spider mites. However, when I treat for spider mites the symptoms persist and then the plant dies. If I leave it alone the plant lives all summer but is ugly. Plus, the green potato plant in the same pot does not suffer the same symptoms.

I wonder if its due to too much sun. When I grow them inside they are a nice color but the leaves are small and the plant is mostly stem. However, I see these plants everywhere. They are in full sun all day in pots outside of restaurants and in the ground outside on manicured lawns; they are gorgeous. The nursery has them in full sun in hanging baskets all day. Why me! :*(

(Answer)

We are somewhat surprised to receive a gardening question from as far away as Texas. It’s good to know that you found our website. Based on the information you have provided we will do our best to provide information on how to care for your purple potato plant which doesn’t look so bad in the photo! Just so you know, Master Gardeners are very active in the United States. In fact there is a Master Gardener program in Texas. Try visiting: www.aggie-horticulture.tamu.edu/contact/ for further information including a section where you can “Ask an Expert”. Your local Master Gardener group will be better able to provide regionally specific advice.

Up here in Toronto we’re in Zone 5-6, which is considerably cooler than Zone 9 in Texas. Ornamental potato vines are popular here but we treat them as annuals. Your potato plant is probably a sweet potato vine ( pomoea batatas). It’s a fast growing shrubby vine native to South America.  They absolutely do well in Zone 9. Ideal for your situation.

Ornamental sweet potatoes are rarely bothered by pests and diseases.  They may succumb to beetles. Cucumber, flea, tortoise and Japanese beetles occasionally feed on sweet potato foliage, leaving holes or skeletonizing leaves in their wake. If you have an infestation of spider mites you should see some webbing. Spider mites do thrive in dry, hot weather – typical in Texas. Maintaining healthy plants by adequately watering and fertilizing strengthens them against insect attacks. If you definitely see insects the least toxic solution to try is spraying with horticultural oil early in the morning.

It’s quite possible your plant is under stress because the container soil is not kept moist enough. Soil dries out faster in containers than in the ground.  Your containers are on the third floor so may be prone to high winds that dry them out even further. If there is a balcony above you any rain you may have will not reach the container. Unfortunately, the symptoms of under watering are the same to those of over watering.  The tips and edges of the leaves turn brown. Try extra watering so that the soil is moist all the time. Do not let it dry out between watering.  At the same time you may want to add a water soluble fertilizer (12-12-12).

If you have had any humid weather you could have a fungus, while on the other hand you may have inherited the problem from the nursery where you purchased the plants. It’s a bit of a mystery, as to why your green plant does well.

You can find more detailed information at:

http://content.ces.ncsu.edu/ornamental-sweetpotatoes-for-the-home-landscape.pdf

http://msucares.com/news/print/sgnews/sg12/sg120716.html