Q. Our Vincas are in pots and have done very well all summer. We have noticed they are starting to be tall/ spindly, and the leaves are beginning to turn yellow. They have been watered three times per week with an auto irrigation system, supplemented occasionally with Miracle Grow from a watering can. Is it the end of the growing season for them, or is there cure?
If you don't know what Vinca looks like
A. The usual problems with Vinca are soil related problems, not enough light and watering too often or the soil not draining well.
Vinca likes to grow in a soil that is well amended down to a depth of about 8 to 12 inches, in full sun and fertilized with a high nitrogen fertilizer once a month. If they are in full sun then your soil is probably running out of organic amendment and you are losing pore spaces in the soil.
The addition of compost to the soil at planting time down to a depth of at least 8 inches is necessary at the time of planting. If you don't add enough compost to the soil, the soil spaces that are created by the compost and soil mix begin to collapse.
As these pore spaces collapse, less aeration reaches the roots and water no longer drains through the soil like it used to. There is not much you can do about that now. It really has to be added at the time of planting. The soil will continue to collapse more and more and then you will start to see isolated plants die.
Upon close examination these plans will have died from root disease or collar rot which is a rotting that occurs right at the soil surface. The other possibilities are lack of light reaching the plants and not fertilizing them on a regular monthly schedule.
You have nothing to lose so try adding a complete fertilizer like Miracle Gro or Peters and see if they perk up. That will not, however, cure a problem if there is not enough sunlight.