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Garden Q&A: How to care for the dipladenia and what to do with zinnias

A dipladenia is shrub variant of the mandevilla family.
A dipladenia is shrub variant of the mandevilla family. (Ellen Nibali/For The Baltimore Sun)

I love this houseplant that blooms year round with red blooms for Christmas, Valentine’s Day, Cinqo de Mayo and even the 4th of July. The label said it’s a shrub mandevilla, but it sends out tendrils like it wants to climb. I cut those off at the base. Is that right? It’s in bright light indoors and outside for the summer with only direct sunlight in the morning. I fertilize spring through summer (randomly) and water once a week unless there is a good rain. It’s in a big pot that we cart indoors into a 50 degree sunroom. My usual approach is either a plant works or it doesn’t, but I don’t want to lose this one.

If it’s blooming year round for you, you are doing it right. Mandevilla is a huge plant family and you may have a dipladenia, a shrub species with more rounded leaves and smaller blooms than the typical vine sold commercially. Both flowers are attractive to hummingbirds and butterflies alike with the same growth requirements.

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Someday you may see roots on the soil surface or coming out drainage holes, which means repotting or pruning. And if you see a white crust on the soil surface, that means a salt build-up that requires replacing the top layer with fresh potting soil. But, as they say, if it ain’t broke, don’t fix it.

I grew hundreds of zinnias this year and hope to do same in the spring. Now that the season is over, is there any benefit to leaving them in place for insects or birds? Should I pull them out by the root?

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Hundreds of zinnias sounds lovely. Some birds, such as finches, enjoy eating zinnia seeds, but seeds may be largely consumed already. Stems of these annuals will deteriorate over the winter, and we do not have research weighing their value for insects over the winter.

However, zinnias often carry powdery mildew and other diseases. Especially since you intend to replant with zinnias in the same spot next year, we would recommend removing the stems and foliage from your garden. You can prune the plants off at the soil line and leave roots in place to deteriorate naturally over the winter and return organic matter to your soil. You also can easily save any seeds still attached to the base of the flower head.

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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