By Jane Malec, EMGV
While volunteering at the Bahama Farmer’s market, an interesting discussion developed on the plight of the once beautiful stand of cannas near the roadside. The leaves looked curled and ratty…so sad.
Cannas are one of the plants we depend on during the warm summer months. With its colorful leaves and huge iris like flowers, they’re a bright spot in the garden. As tropical plant, they will stand up to the heat plus they are not at the top of the deer menu! Also, these rhizomes are generally easy to grow.
Although they are largely free of pests, there are two that you need to look out for. Canna leaf rollers can be a real problem and is what we uncovered in the large bed at the farmer’s market. Stalk borers can also cause big headaches.
The leaf rollers, also known as Brazilian skippers, chew straight rows of holes on canna leaves. Some rollers feed in groups under a netting of their own silk. The solitary types feed by rolling, folding or tying leaves together before they eat them. This last process is actually very interesting as it looks like the leaves have been sewn with a needle and thread. Leaf rollers are active at night while sleeping on the underside of the leaves during the day. The damage to young leaves usually prevents them from opening and they may die. Infested plants can’t bloom and are ugly. Its easy to miss the first generation of these caterpillars if you aren’t monitoring the rolled leaves carefully. The second and third generations will be more obvious because by then the damage will be extensive.
If you are able to detect these vile caterpillars early enough, you can cut the leaves and destroy them or wash them off. Unfortunately, washing them maybe unproductive if there are many leaves as we found in Bahama. You can also use a systemic insecticide such as a 3-in-1 for roses. There are other options including insecticidal soap. Bacillius thuringienis is also effective if applied to the sticky underside of the leaves. Even a household detergent may work if applied to the leaf underside.
Stalk borers have a broad host range upwards of over 200 species of plants. This little villain can go after cannas, tomatoes, goldenrod and even some fruits. Sometimes even twigs of some trees are victims. The first sign that you have stalk borers in your canna will be wilting leaves but by this time, its often too late. You can try to kill the borer by cutting into the infected stem and digging it out. Your best bet is to remove the infested plant and then kill the borer. This problem is often sporadic and thereby difficult to predict which plant will succumb. Please check the second link at the bottom for greater detail on this pest.
Here comes some very important advice for both of these insect problems…discard all of the plant material after die back no matter when that is! DO NOT COMPOST! The leaf roller and stalk borer larva will overwinter in dead leaves. If you have a big stand of cannas and don’t clean up the debris throughly it will most likely present an even bigger problem next season.
There will also be the occasional slugs, snails, thrips and mites to deal with and should be treated as you do with another plants.
My cannas are in a pot and are three years old. So far I have not experienced either of these little menaces but I am inspecting the leaves very closely now.
http://www.aces.edu/pubs/docs/A/ANR-1315/ANR-1315.pdf
https://www.extension.umn.edu/garden/insects/find/stalk-borers/