I am trying to grow London Plane trees from seed taken from a tree in Boston. I managed to germinate some seeds last summer and have a nice 18″ specimen under grow lights as well as a second bunch that are about an inch from a second planting. I was planning on putting the larger one in the garden this spring. The larger tree’s leaves are turning brown and I was wondering if the tree requires a period of dormancy or whether it is suffering from a disease. I have been spraying it with sulphur based fungicide but it doesn’t appear to be helping. I have also noticed small insects on both the top and bottom of some of the leaves, but predominantly the bottom. The seedlings are all growing in potting soil (versus outdoor garden soil), and have not been fertilized.
Thank you for contacting Toronto Master Gardeners regarding your London Plane Tree (Platanus x acerifolia, Platanus x hispanica) seedlings. Based on the information provided, it appears your seedlings have scale insects. They puncture plant tissues and feed on the sap, which can cause small holes and wilting of the leaves. You may also notice a sticky substance on the leaves called honeydew, which is a byproduct of their feeding activity. Since your seedlings are still relatively small, you can likely control the insects by hand picking. The insects typically do not live in the soil, but they can move between plants, so be sure to inspect the other seedlings and any other plants in your house, and isolate the affected plants to prevent the insects from spreading to unaffected plants. If hand picking is unsuccessful or not possible, there are insecticidal soaps that will control the insects at their immature stage, but you will need to check the label to make sure they can be used indoors. Control of adults requires a different product such as horticultural oil, but again, you will need to verify that the product you choose is safe for indoor use. My suggestion would be to start with the simplest approach first (i.e. hand picking) then consider other options if that proves unsuccessful.
You can discontinue use of the fungicide as it is probably not necessary. When planting the seedlings outdoors, be sure to harden them off first by gradually exposing them to outside conditions over the course of one or two weeks in late spring after all danger of hard frost has passed. Remember, they have enjoyed a comfy life indoors and have never been exposed to the outdoor elements. Start with just an hour in a sheltered spot on the first day. Increase the time outdoors by about an hour a day, until the seedlings are spending 7 or more hours outside. At that point, you should be able to leave them outside permanently and plant them in the ground. If you do not plan on planting them right away, be sure to keep them in a sheltered location and check the pots regularly, as they will dry out faster than when they were indoors. The seedlings will undergo dormancy in the fall, once they are growing outdoors and are exposed to seasonal changes in light and temperature.