Tree dahlia (Dahlia imperialis) with leaf miners

Answer from Pat:
The plant you are growing is a Tree Dahlia (Dahlia imperialis), a plant that amazes and amuses gardeners and non-gardeners alike. It’s a novelty from South America that starts out as a small plant in spring and shoots up perhaps as tall as an eye-popping 20 feet in one year, bursting into bloom in fall (usually October or November) with pink or white blooms that come in single flowers or doubles.

When full grown the stems look like bamboo. If planted where folks can see it, they sometimes stop their cars to gawk or snap a photo.  Since your plant has leaf miner and you have a drought-resistant garden with invasive tree roots, my guess is that your plant is stressed and this explains why it got leaf miners. Leaf miners are usually controlled by beneficials in organic gardens, but once they lay their eggs on a plant and larvae enter the leaves they cannot be killed with sprays like Pyrethrum or Spinosad which are not systemic. It sounds to me as if your plant needs fertilizer and more frequent and deeper watering. The best ones I have ever seen were not given any special care but they were growing in regularly irrigated, fertile flower beds and away from roots of invasive trees which can sap all the goodness out of the ground and starve other plants of nutrients and water.

Regarding leaf miners: One never sees leaf miner as a flying insect since they are very small.  One only sees the wandering tracks of the larvae inside the plant. I have many times during the years faced the depredations of leaf miners and once they appear the best thing one can do is take such good care of the plant that it shrugs them off. Yes, you can cut off the damaged leaves but the plant needs foliage or it won’t be able to grow. I once planted a tree dahlia that got eaten by snails, so don’t be too discouraged. I was a total failure at growing this plant, through not protecting it from snails.

Most likely your plant won’t die from leaf miner, only be weakened by them, and that will be a temporary condition if you provide it with what it needs to fight them off. Take off the worst leaves so leaf miners can’t proliferate, but don’t completely denude the plant.

If I were you I would increase irrigation for this one plant. By August, which is now, your plant should be three or four feet tall if growing in well-irrigated bed and remember in order to support all this rapid growth your tree will have to spread its roots deep and wide, so make sure you are watering and fertilizing a large enough area. For a fertilizer: aged chicken manure would be good if you have it. If you have chicken manure that is not yet aged, just dilute some in water to make a liquid fertilizer and pour that around the roots. Additionally, water your plant deeply at least once a week, but twice a week would be better. (Alas, Tree Dahlia is not drought-resistant.) Home made compost could help since it creates good soil conditions and these can be picked up by the plant. Spraying with anything other than a systemic spray will do nothing for the plant but I have an idea of something that might help.

After fertilizing, cover the ground under the plant with a 1- to 2-inch thick layer of dry earthworm castings, and water this into the ground. By watering to wash the earthworm castings into the ground you enable the roots of your Tree Dahlia to pick up the chitinase that is contained in them. Chitinase attacks the chitin in insects. (The exoskeletons of insects are made of chitin.) Thus, increasing the amount of chitinase inside the plant can help fight off the leaf miners and might even kill them.

Good luck with your Tree Dahlia!