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Hortiscope: Black knot disease can be prevented with spray

Q: I have read your comments about black knot disease on Canadian cherry trees. What routine care do you recommend to avoid this disease? The information I have on the tree says it likes sandy soil. I have very hard clay soil. Should I add sand to the soil when I plant the tree? If so, what ratio of clay and topsoil should I use? I never have grown trees before. Are there certain basics I should know about?

A: The black knot fungus may not be as serious in your area as it has been in our part of the country. As far as the cherry tree needing sandy soil, it does not. We have thousands of them growing in Red River Valley clay. To prevent black knot disease from getting started, spray the tree before and during blooming with sulfur, captan, thiophanate methyl or any fixed copper fungicide that lists chokecherry on the label.

Not all of this may prevent the fungus spores from getting started on your tree. If the knots should start to show up, carefully prune them out by going back at least 6 or more inches from the visible site of infection. If necessary, cut the entire branch off to avoid leaving a stub. Basic tree care involves watering deeply when needed. On a new or young tree, it is critical to pay attention to this more so than when the tree becomes established. The soil should be kept moist but not soggy so the tree does not suffer drought/heat damage.

Q: I have learned much about my hibiscus tree. I was planning to repot it until I read not to because it loves to be root-bound. How do I know when it is too root-bound? There is a 1-inch root emerging from the top of the pot. I added 3 inches of potting soil and some petunias. Is this OK?

A: It shouldn't hurt a thing. By the end of this growing season, I would suggest taking the petunias out and repotting the tree into the next larger-sized pot.

Q: I was given a hybrid lily plant as a door prize. It had a beautiful orange flower on it that looked almost like a tiger lily. A couple of days after I got it home, it started to look like it was dying. It came in a very small pot, so I transferred it to a larger pot. That didn't help. Now all the leaves are dying. Is there anything I can do?

A: Move it outdoors into the garden to see if it recovers. Lilies usually are tough as nails, so don't be quick to give up on it. However, don't pamper or push it along with too much water or fertilizer. If it is going to make it, it will do so on its own.

Q: I have conflicting reports as to when the best time is to move or take a root from a fern peony. One source says spring is the best time, while another says fall. I'm confused.

A: Depending on where you live, peonies usually go dormant by late July or August. When finally dormant is the time for you to divide or take a root.

To contact Ron Smith for answers to your questions, write to Ron Smith, NDSU Department of Plant Sciences, Dept. 7670, Box 6050, Fargo, ND 58108-6050 or e-mail