How Cold Is It Before Penta Flowers Freeze?

Penta flowers (Pentas lanceolata) produce small, brightly colored, star-shaped flower clusters in warm regions, such as those in U.S. Department of Agriculture plant hardiness zones 8 though 11. While penta, or star, flowers are evergreen perennials, they are grown as houseplants in USDA zones 8 and lower, where temperatures dip below freezing. Outdoors these plants are useful for borders and containers, and they bloom almost year-round within their hardiness zone.

Penta flowers cannot withstand a hard freeze. This means they need proper care in areas where temperatures drop below freezing to the upper 20 degree to lower 30 degree Fahrenheit range and are unlikely to survive temperatures in the mid-20s F and lower. When temperatures are predicted to dip to these lows, you must take steps to protect the plants -- unless you are growing penta flowers as annuals and plan to replace them the following spring.

To protect outdoor penta flowers, you can place them in pots and bring them indoors for the duration of the freezing temperatures. Alternatively, you can take measures, such as mulching or covering the plants. Mulch only protects the crowns, stems and roots of the plants, while covering them protects the tops as well. To protect an entire plant, cover it with canvas or plastic fabric large enough to extend to the ground while covering the plant. Stake the fabric down to trap the soil's warmth, and remove the cover during sunny, mild days.

During winter penta flowers growing in USDA zone 10 or above rarely face freezing temperatures. However, in early spring, you can cut the plant back hard, which means cutting the plant down to ground level. This helps encourage new growth. Regular deadheading during the growing season increases bloom numbers and extends blooming time.

When grown as houseplants, penta flowers require a minimum temperature of 55 F in a bright area of the home. Place potted penta flowers in a container with a growing medium of equal parts leaf mold, peat and loam. When plants outgrow their pot, you can repot them in March using a pot two sizes larger than the current pot. If you move penta flowers outdoors, do so eight to 10 weeks after the last frost, when soil has warmed to at least 50 F.