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Garden Help Desk: How to better prepare your petunias

By Meredith Seaver - USU Extension | Oct 3, 2021

Petunias are a popular flowerbed plant, but growing them in the same place year after year can lead to a buildup of "petunia diseases" in the soil.(Courtesy Meredith Seaver)

I’ve always had petunias in my flower beds, and they’ve always looked beautiful, but last year I noticed they weren’t quite as nice, and this year some of the petunias died. Is there a spray or fertilizer I can use to fix it for next year?

When you grow the same annual plants in the same place year after year, diseases can build up in the soil. After a few years of repeating the same plantings, it’s common for plants to succumb to the disease. This can happen in vegetables gardens as well as in flowerbeds and container gardens.

If the problem is a buildup of disease in your flowerbeds’ soil, there isn’t a spray or fertilizer that will solve this problem and let you successfully grow petunias again next year. The solution for this problem is to rotate through a few different unrelated plants in your flowerbeds before you try petunias again. If you really want to include petunias in your landscape and flowerbed designs, consider including containers in the “off years” instead of planting them in the ground.

You can also add some plant-based compost to the soil. This can add beneficial microorganisms to the soil and help reduce the disease load for some diseases.

Is this the bug that’s killing my tree? My walnut tree has been dying back for a while, like some other walnut trees in my neighborhood. I found this bug on the tree. Is this what’s killing the trees on my street?

The cankers from Thousand cankers disease will be very small at first, but they will expand as the disease progresses. (Courtesy Meredith Seaver)

This insect is a Pigeon tremex, a type of non-stinging, wood boring wasp. The tremex in your photo is a female and that scary looking “stinger” is her ovipositor, used for laying eggs under the bark of hardwood trees.

The Pigeon tremex is harmless to people and healthy trees. Females are attracted to dead, dying or severely stress hardwood trees. While it’s not impossible, walnut trees aren’t the preferred host for the Pigeon tremex, so there may be other declining trees like maple or ash trees in your neighborhood. Since trees attacked by the Pigeon tremex are drying or dead, there isn’t any real benefit to trying to control them.

The problem with the walnut trees on your street is more likely related to a different insect- a tiny beetle called the Walnut twig beetle and the fungal disease it carries- Thousand cankers disease. Black walnut trees are very susceptible to this disease, but it can also affect English (Persian) walnut trees.

Walnut twig beetles may be tiny, but they usually infest trees in large numbers and the fungal disease they carry into a tree causes cankers in the cambium and phloem under the bark at every twig beetle feeding site. Cankers merge and eventually kill branches and or trees. Once you can see dying branches with cankers, the tree will continue to decline and may die within just a few years.

Is it a good time to start cutting back hostas and daisies, or should I wait? If I should wait, for how long?

This English walnut tree has been infected by Thousand cankers disease. Branches will continue to die until the tree either dies or is removed. (Courtesy Meredith Seaver)

Plants spend their time in the sunshine making carbohydrates. Some of those carbohydrates get used for growth during the season and some of them are stored for growth when your plants break dormancy in the spring.

Some plants begin to slow down and go dormant early in the fall while others will be green and active for several more weeks. If your daisies and hostas still have some nice green leaves, then they’re still making energy that can be stored for next year and your plants will benefit if you wait a little longer before you cut them back. If the leaves and stems are yellow, brown, or breaking down, then your plants have finished their work for the season, and you can cut them back for the winter.

If you’re pressed for time because you’re going to be away from home, very busy at work or because of something else that will be filling your schedule soon and keeping you out of your garden, it’s fine to cut back the plants now and tidy up your flower beds. Your plants have already stored some of the energy that’s been produced the past few weeks, so you’ll only lose a few days or weeks of benefit from those leaves.


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