Vernalizing Biennials in Storage

Learn the proper way to vernalize biennial plants in storage.

By Micaela Colley & Jared Zystro

The Seed Garden

The Seed Garden (Seed Savers Exchange, 2015) by Micaela Colley & Jared Zystro and edited by Lee Buttala & Shanyn Siegel brings together decades of research and hands-on experience to teach both novice gardeners and seasoned horticulturists how to save the seeds of their favorite vegetable varieties.

You can purchase this book from the MOTHER EARTH NEWS store: The Seed Garden.

Understanding what conditions are necessary and how they can be met is the first step in consid­ering the possibility of vernalization in storage. Most biennial vegetable crops require a period of 8 to 10 weeks at temperatures below 50°F (10°C) in order to vernalize. They will need to be dug and stored in a manner that simulates these con­ditions in regions that do not get cold enough or in locales where temperatures get too cold. While plants can be stored at temperatures anywhere between 32 and 45°F (0 and 7°C), a temperature range between 34 and 37°F (1 and 3°C) is rec­ommended both to prevent damage from freezing and to help prevent rot and disease that may occur at warmer temperatures.

A high relative humidity is also desirable for storage—with the ideal for most crops being 95 percent relative humidity, but relative humidity levels as low as 75 percent should be acceptable. Managing the moisture level of the storage substrate can help provide the desired relative humidity. The goal is to manage humidity levels so that plants neither desiccate nor rot. Traditional root cellars and refrigerated storage come close to providing this temperature range and these levels of relative humidity and are the most adequate stor­age locations. However, many seed savers simply do the best they can to provide some approxima­tion of these conditions—in an unheated garage or outbuilding—and succeed in keeping plants alive throughout the winter and producing seeds in the second season.

Crops that will be vernalized in storage are typically dug up as late in the season as possible, taking into consideration both the temperature of the place where they will be stored and the possibility of the ground freezing or the plants being damaged by the cold before they can be dug. Ideally, plants should be harvested after they have reached an appropriate size and when the

soil is slightly dry to avoid excess soil clinging to their roots. By brushing or rubbing off excess soil as opposed to washing the roots, a seed saver can discourage disease and rot during storage.

Leaves and roots should be trimmed to vary­ing degrees, depending on the crop. The leaf stems of most root crops are trimmed on a diag­onal to form a small point about one-half inch (1.3 cm) from the crown. Cutting too close to the crown could damage the growing point, but the removal of leaves and stems is beneficial because it helps prevent rot during storage and reduces loss of moisture in the storage tissues. Leafy plants, such as kale and collards, are usually trimmed of most of their foliage and then left to dry for a day in a cool location away from direct sunlight. When trimming leaves, it is critical to leave the main growing point intact. In the case of collards and kale, the growing point is located at the top of the main stem.

Root crops, such as carrots and beets, can be stored in perforated plastic bags or in crates with slatted sides that allow adequate air circula­tion. Roots should be layered in containers and kept from direct contact with one another using wood shavings, shredded leaves, peat moss, or moist sand. Crops with fibrous root systems, such as kale, celery, collards, or Swiss chard, should be replanted into slightly moist soil, potting mix, or sand in clean nursery containers. Depending on how they are stored, plants or roots should be monitored periodically while they are in storage, and any showing signs of significant rot or disease should be discarded.

As a general rule, plants can be set out in the garden in the spring at the desired spacing once the ground is workable and temperatures are appropriate. Plants should be watered and culti­vated as needed to reestablish in the garden prior to flowering and seed set. 

At Heritage Farm in Decorah, Iowa, rutabagas are sown in late July, so they will be approaching market maturity in October when they will be harvested and moved to a root cellar for overwintering and vernalization.

vernalization biennial
vernalization biennial
In late fall, the tops of rutabagas are cut back in the field, prior to digging. Rutabagas are lifted from the ground and brushed clean of soil and debris.
vernalization biennial
vernalization biennial
vernalization biennial
The roots are layered in perforated con­tainers in a mixture of peat moss and wood shavings, and then stored through the winter in a root cellar. In the spring when the ground is workable, typically in early April, plants are taken out of storage. Rutabagas are transplanted back into the field with 18 inches (46 cm) between plants in rows 4 feet (1.2 m) apart.
Mulched with straw for weed control and moisture retention, the second-year plants put on new vegetative growth prior to flow­ering. Rutabagas are often one of the first biennials to flower in spring, and in Iowa, seeds are typically harvested in July.

For more on seed saving, see our Seed Saving Guide.

Reprinted with permission from The Seed Garden, by Micaela Colley & Jared Zystro, edited by Lee Buttala & Shanyn Siegel and published by Seed Savers Exchange, 2015. Buy this book from our store: The Seed Garden.

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