Tips for keeping Mexican petunias and nutgrass in check.

Q: Could you tell me how to kill the roots of Mexican petunias? They are so pretty, but they have taken over my yard.

Joanne Geller, Richmond

A: I agree Mexican petunias (Ruellia brittoniana / Ruellia simplex) are pretty, especially when grown in a tight clump that creates a pool of color. But I also find both the tall and compact forms extremely vigorous. Without a gardener’s watchful eye, these prolific reseeding, root expanding perennials quickly can outgrow their welcome in the landscape.

You’ll need patience as you work to gain control:

Dig up the roots of established plants. Make sure to get the entire root system, or soon you’ll be digging again. I’ve found digging is much easier in soil I’ve previously enriched and loosened with compost.

Pull young plants that have sprouted from seed as soon as possible. Keep an eye out for new volunteer plants.

Apply a herbicide on the foliage to kill the top of the plants. Avoid contact with nearby plants. Reapply when new growth emerges. Roots can’t continue without top growth and will eventually die.

Q: Our croton is leafy and nice in a partial-shade area. However, now most leaves are green. Some are yellow, but none are red anymore. What should I do so the plant will produce red leaves again?

Terry Li, Houston

A: The new leaves of numerous croton cultivars emerge green and yellow and turn shades of red, orange or salmon as they mature. Some cultivars offer other colors. Leaf shapes also vary.

These vivid foliage plants are best in bright light, so perhaps moving it to a spot with more sun will help. Mature green leaves likely won’t change, but new foliage might colorfully age.

Q: Will you repeat the proportion of molasses/water for killing nutgrass? I failed to write it down.

Harriet Eisele, Seabrook

A: If you’re weary of digging up this pest, organic gardeners, including the Dirt Doctor Howard Garrett, recommend molasses to control nutgrass.

Garrett’s recipe is similar to others: Drench troublesome areas with ¼ to ½ cup of liquid horticultural molasses per gallon of water. Apply a gallon of drench per 10 square feet. The molasses energizes soil microbes that work on the nutgrass bulblets. Two or more applications are needed to get the job done.

Q: I’ve been wondering why my fringe plants, or loropetalum ‘Plum Delight,’ have not bloomed in more than a year. The shrubs are more than 5 years old, and I keep them trimmed like a hedge along the backyard (full sun) flower beds. I fertilize only slightly because they really were hardy at the beginning. Is it possible these shrubs have only a few years that we would enjoy those beautiful blooms and they stop after a certain number of years?

Annabel Harris, Houston

A: No, with proper culture, loropetalum continues to flower. It’s possible that in repeatedly trimming these fast-growing shrubs as a hedge, you’re removing future blooms. Loropetalum sets buds in summer that become late-winter to earlyspring blooms. While loropetalum takes well to pruning, trim right after the plants bloom to avoid bud loss.