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You can try cutting them back to the ground and see if they will regrow. Make sure there is no more frost coming before you cut them back. Mexican heather, however, does not tolerate a hard frost, so you may have lost them.

https://www.gardeningknowhow.com/ornamental/groundcover/false-heather/growing-mexican-heather-plants.htm

Mexican heather is not hardy in your zone 7 so it shouldn't be a problem. It will die out with the first hard frost.

https://www.gardeningknowhow.com/ornamental/groundcover/false-heather/growing-mexican-heather-plants.htm

Mexican Heather (Cuphea hyssopifolia) is a warm-season annual or perennial plant that can be grown in USDA hardiness zones 8b and above. In Zone 8b, which typically has mild winters, you can plant Mexican Heather during the spring or early summer when the temperatures have warmed up and the risk of frost has passed.

Here's a general guideline for planting Mexican Heather in Zone 8b:

Soil Preparation: Prepare well-draining soil with organic matter to improve moisture retention. Mexican Heather prefers slightly acidic to neutral soil.

Sunlight: Choose a location that receives full sun to light shade. Mexican Heather performs best in bright, indirect sunlight.

Planting Time: Wait until the last frost date in your area has passed before planting. In Zone 8b, this typically occurs in late winter or early spring. I would try and keep them safe until then.

Planting Depth: Dig a hole that is as deep as the root ball and slightly wider to accommodate the plant. Space multiple plants about 12 inches apart.

Watering: Water your Mexican Heather regularly to keep the soil consistently moist but not waterlogged. Once established, it is somewhat drought-tolerant but benefits from regular watering during dry spells.

Mulching: Apply a layer of mulch around the plants to help conserve moisture and regulate soil temperature.

Fertilizing: Use a balanced, slow-release fertilizer or a fertilizer formulated for flowering plants to promote healthy growth and abundant blooms. Follow the recommended dosage on the fertilizer label.

Pruning: Mexican Heather can benefit from occasional pruning to encourage bushier growth and to remove spent flowers.

Remember that the specific planting and care requirements may vary slightly based on your local climate and soil conditions, so it's a good idea to consult with a local nursery or garden center for more personalized advice.

https://www.gardeningknowhow.com/ornamental/groundcover/false-heather/growing-mexican-heather-plants.htm

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