Yard and Garden: Over-wintering potted plants

AMES — It’s hard enough to keep perennials in the ground alive during Iowa’s extreme winters. It’s much harder to over-winter potted trees, shrubs and plants.

The bulk soil in the ground tends to moderate the temperatures; the small amount in pots tend to give up heat more readily. Horticulture specialists with Iowa State University Extension and Outreach offer several ways to over-winter potted plants. To have additional questions answered, contact Hortline at hortline@iastate.edu or 515-294-3108.

How can I over-winter
container grown trees and shrubs?

Container grown trees and shrubs should not be left outdoors above ground over the winter months. The roots of most trees and shrubs are far less cold hardy than their aboveground stems or trunks. Container grown plants are growing in relatively small amounts of potting soil. The temperature of the potting mix may drop into the single digits if container grown plants are left above ground in winter. Single digit temperatures may damage or destroy the root systems of plants.

An excellent way for home gardeners to over-winter container grown trees and shrubs is to dig holes in a garden area and set the pots in the ground. After the plants have been placed in the holes, place soil around the pots as if planting them. Soil is a good insulator and will protect the plant’s roots from extreme cold.

Container grown trees and shrubs can also be over-wintered by placing them in a moderately cold location (temperatures from 20 to 45 degrees Fahrenheit) over the winter months. The cold temperatures will keep the plants dormant until spring. An attached, unheated garage is often a suitable over-wintering location.

Container grown perennials can be over-wintered in a similar manner.

I have a miniature rose
growing in a pot outdoors on my patio. How do I
over-winter it?

A potted miniature rose can be kept as a houseplant through the winter months. Bring the plant indoors before a hard freeze. Place the miniature rose in a sunny window or under artificial lighting. Avoid sites near cold drafts or heat sources. Water the plant when the soil surface becomes dry to the touch. Apply a dilute fertilizer solution every two to four weeks through the fall and winter months.

Another option would be to over-winter the miniature rose outdoors. In early November, dig a hole in a protected garden location. Place the potted rose in the hole. Then place soil around the pot. Several days later, cover the bottom 8 to 10 inches of the rose with additional soil. The soil should remain in place through the winter months. In early April, remove the soil around the canes, carefully dig up the potted rose, and prune out any dead wood.

How do I over-winter the common fig?

Common or edible figs (Ficus carica) are not reliably cold hardy in Iowa. The stems of ‘Chicago Hardy’ (considered one of the hardiest edible fig cultivars) are hardy to 10 degrees Fahrenheit and the plant’s roots are hardy to 15 to 20 degrees below zero. In Iowa, common figs are best grown in large containers and over-wintered in a cool location, such as an attached, unheated garage or cool basement.