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| Photo Credit: Jenny Hooks |
| Before a sudden cold spell in Arizona, this hibiscus was doing great. |
Why do plants die in winter? There can be many causes. Let’s first explore how plants ordinarily react to low temperatures.
Evergreens and deciduous plants adapt to cold differently. If you have evergreens that have dropped all their leaves, that’s a serious problem. For deciduous plants, it’s completely normal. Some evergreens survive pretty cold temperatures, such as conifers or cone-bearing plants, but many plants that thrive in subzero-degree climates are deciduous species. Some plants fail to survive because they don’t “harden-off,” or adjust to cold temperatures. Tropical hibiscus would not be expected to survive below-freezing temperatures; on the other hand, hardy hibiscus – found as shrubs and small trees – generally lose their leaves as the days shorten and go dormant before winter arrives. Plants sometimes never harden-off due to cultural practices; if they were pruned late in summer or were overfertilized in fall, the cold can catch them unprepared for winter. This often happens with roses. Underfertilized plants can suffer as well. If they don’t have enough carbohydrates stored in their stems and roots, they can fail to form buds in the spring. If plants are small and recently planted, they might not have had time to establish good root systems by winter. Has it been cold enough for the ground to freeze? Freezing and thawing can dislodge your plants (since there’s no root system to hold them down), sometimes causing them to pop out of the ground – a condition called heaving. And were the plants watered immediately after they were planted? Watering-in is a crucial element to good plant establishment. In dry winters, evergreens should be given adequate water; root systems don’t go dormant and need moisture to survive. Finally, there’s what is called provenance, or the origin of the seed source of the plant. For example, a plant native to the Upper Midwest – let’s say bur oak – would not do well in the Deep South because it would not receive adequate cold in the winter (chilling); a bur oak native to south Louisiana likely would freeze in Fargo.
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