Garden Q&A: Protecting dahlias

For The Baltimore Sun

My dahlias are still blooming like it’s summer. How long does this go on? Since they can’t handle winter, how do I protect them?

Dahlias are jewels in the garden until frost. Do not wait until soil freezes to dig up the tubers or you’ll be left with mush. After the first frost or when leaves yellow, work with a spading fork or shovel, loosening soil around them, then removing tubers gently. Avoid cutting tubers, as rot can set in. Clinging soil should not be washed off. Cure by spreading and drying them in shade for one to three days. Store in sawdust or peat moss, slightly dampened, in a cool damp place such as a basement or garage at about 50-55 degrees. Check occasionally and dampen if they appear to be shriveling.

Starting about mid-summer the last few years, our pin oak’s leaves start browning along the edges. At first, it was only some of the lowest leaves, but now more leaves are doing it. The tree looks great each spring, though now a few of the low branches are dead. Is this normal or serious? I’d say one-third of the tree is now affected.

Bacterial leaf scorch is a serious disease, usually slow-developing, with no cure. The bacterium is spread by common insects such as plant hoppers. It infects low interior leaves first, moving outward to branch tips and progressing upward into the crown. (A drought problem, on the contrary, would start at the top and progress downward.) Dead branches should be removed. Though antibiotic treatments and fertilization have been shown to prolong the life of infected trees, an infection as advanced as your tree’s would not be helped. Search bacterial leaf scorch on the HGIC website. Oaks are of prime importance in the ecosystem. Consider replacing your pin oak, a member of the red oak family, with an oak in the white oak family, which is more resistant to bacterial leaf scorch and more drought tolerant.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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