Evaluate Cold-Tender Plants Before Overwintering to Avoid Future Problems

The looming threat of the first hard frost is inescapable at this point of the year across Iowa. Folks who enjoy and grow tender perennials and annuals should be preparing for overwintering those tender perennials. Before jumping to the “how-to overwinter tender perennials” article, evaluating the health of these plants is important. Many gardeners check their plants for insects like mealybug, scale, whitefly, etc. before bringing them in to overwinter. But carefully observing your plants can save you from the negative effects of keeping plants that have incurable or difficult-to-control diseases. 

Abnormal foliage: spots, rings, and distortions

There are several reasons why foliage may look abnormal. The abnormalities of greatest concern are those that may be caused by disease. Some diseases are common in plants and may or may not affect plant growth the following year. However, viral infections are something we want to avoid. While some viruses cause minimal damage, others can significantly impact plant health and spread to other plants.

One important thing to keep in mind is that once a plant is infected with a virus, it cannot be cured and will remain infected for the rest of its life. Although laboratory testing is the only way to confirm the presence of a virus, any foliage exhibiting unusual symptoms should be considered suspect. If a virus is present in the foliage, it will also be present in the tubers, rhizomes, or cuttings of that plant.

Removing and discarding affected plants is important because these plants are unlikely to thrive and may potentially spread the virus to other plants. Some viruses affect only specific groups of plants, while others have a very broad host range. Therefore, if you suspect a plant is infected with a virus, it is best to dispose of it rather than invest the time and effort required to overwinter it.

Abnormal Growth: galls

Plant galls are a type of abnormal plant growth. Galls can be caused by many different factors, including injury, infection, insect larvae, and other organisms. The gall of greatest concern in horticulture is crown gall. Crown gall is caused by a bacterium and often develops on plant wounds or near the base of the plant. The galls vary in appearance but often have a texture similar to a walnut shell.

Once a plant develops crown gall, there is no cure. Crown gall is of particular concern because it has an exceptionally broad host range and can spread to susceptible plants through contaminated tools, including pruning equipment. Although galls are undesirable on any plant, they are especially problematic on woody ornamentals and fruit crops, where they can gradually girdle stems and reduce vigor, potentially damaging plants that have taken years to establish. Other gall-forming diseases, such as leafy gall, may also cause significant growth distortions and plant decline.

Crown gall and other infectious galls can be somewhat challenging to identify without laboratory confirmation, but it is good practice to discard plants with crown gall-like symptoms rather than attempting to overwinter them.

If you suspect a plant has a viral infection or an infectious gall:

• Discard the plant. Overwintering plants requires time and effort, so focus your resources on healthy plants that are more likely to thrive next season.
• Do not propagate plants suspected of having viral infections or infectious galls, as these diseases can be spread through cuttings, divisions, and other propagation methods.
• Although composting is generally encouraged, some viruses and the bacteria that cause infectious galls can persist in plant material. To reduce the risk of spreading these pathogens, dispose of suspect plants in the trash rather than adding them to a compost pile.

If you have any plant health questions, you can always reach out to your local ISU Extension & Outreach office or the ISU Plant & Insect Diagnostic Clinic.