Why Potted Cold Hardy Fruit Plants Need Winter Protection

As container gardening with annuals has grown in popularity, more gardeners are also experimenting with perennial fruit crops in containers. Dwarf apples, blueberries, and other compact fruit varieties have been developed specifically for this purpose. While these container-friendly plants offer space-saving and attractive options, it's important to provide them with winter protection.

Many of these fruit crops are fully cold-hardy across all of Iowa's hardiness zones. However, when grown in containers rather than in the ground, their cold tolerance can be significantly reduced. Why is that?

To explore this, I looked at four locations across Iowa—each with weather stations that record both air and soil temperatures (Figure 1):

• Northwest: Ocheyedan
• Northeast: Nashua
• Southwest: Greenfield
• Southeast: Crawfordsville

Table 1 shows the days when air temperatures dropped to -15°F or lower, along with corresponding soil temperatures at 4" and 12" depths. Notably, the 4" soil temperature never dropped below 7°F, and the 12" soil temperature stayed above 24°F. This demonstrates how soil acts as a buffer against extreme cold—much like how root cellars were used before modern refrigeration.

These data clearly show that the rooting environment in the ground is much less extreme than above ground. Roots are generally less adapted to the intense cold and dry conditions that occur during winter above ground.  Shallow rooted plants like sedum are likely more adapted to extreme soil temperatures than our fruit crops that are deeper rooted.

To further illustrate this, Table 2 shows the difference between air temperature and soil temperature on those same cold days. On average, the 4” soil temperature was 37°F warmer than the air, and the 12” soil temperature was 46°F warmer.

This raises an important question: How cold does the root zone of a 12” pot get on a night when air temperatures drop to -15°F? While it may not reach the exact air temperature, an exposed container will likely get very close. In a mild winter, an unprotected container might survive, but it’s not a risk worth taking.

Winter Protection Options

To protect containerized fruit plants during winter, consider these options:

• Move containers to an insulated but unheated space, such as a garage or porch. These areas can buffer against extreme cold while still allowing the plants to meet their chilling requirements for dormancy. Keep in mind that warmer daytime temperatures in these spaces may cause plants to break dormancy earlier in spring.
• Group containers together and insulate with mulch, such as straw or wood chips. This helps trap heat and mimics the buffering effect of soil, protecting the root zone from extreme temperature swings.

Either method can help ensure your containerized fruit plants survive the winter and thrive in the growing season ahead.