This comprehensive guide delves into the crucial process of vernalizing biennial plants during their storage period, a vital step for successful seed production in their second growing season. It outlines the specific environmental conditions necessary, such as precise temperature and humidity levels, and offers practical advice on how to prepare plants for storage. By following these recommendations, gardeners and seed savers can significantly enhance the survival rate of their biennial crops through winter, ensuring a bountiful yield of seeds. The text emphasizes the importance of mimicking natural cold conditions while preventing damage from extreme temperatures or disease, providing a robust framework for preserving these valuable plants.
For successful vernalization, most biennial vegetable crops require a sustained period of cold. Specifically, they need eight to ten weeks at temperatures consistently below 50°F (10°C). When natural climatic conditions don't meet these requirements, or if temperatures dip too low, plants must be harvested and stored in a controlled environment that simulates these conditions. The optimal storage temperature lies between 34 and 37°F (1 and 3°C), as this range effectively minimizes both freezing damage and the proliferation of rot and diseases that can occur in warmer settings. While temperatures between 32 and 45°F (0 and 7°C) are generally acceptable, maintaining the narrower recommended range offers superior protection and plant health.
Beyond temperature, a high relative humidity is also paramount for effective storage. Ideally, most crops thrive in conditions with 95 percent relative humidity, though levels as low as 75 percent can still be sufficient. Properly managing the moisture content of the storage medium is key to achieving the desired humidity. The primary objective is to strike a delicate balance: prevent the plants from drying out (desiccation) while simultaneously avoiding excessive moisture that can lead to rot. Traditional root cellars and modern refrigerated storage facilities are excellent choices as they naturally approximate these ideal temperature and humidity conditions. However, even gardeners without access to such sophisticated setups often succeed by utilizing unheated garages or outbuildings, adapting these spaces to best simulate the required environment, thus allowing their plants to survive the winter and produce seeds the following year.
Preparing biennials for storage involves careful timing and technique. Plants slated for vernalization should typically be harvested as late in the season as feasible, with consideration given to the impending storage temperatures and the risk of ground freezing or cold damage prior to digging. Ideally, plants should be harvested once they've reached an appropriate size and when the soil is slightly dry. This dry soil condition helps prevent excessive soil from adhering to the roots. Instead of washing the roots, which can introduce moisture and encourage disease, excess soil should be gently brushed or rubbed off to minimize the risk of rot during storage.
Trimming the foliage and roots is another crucial step, with the extent of trimming varying by crop type. For most root vegetables, the leaf stems are cut diagonally, leaving a small point approximately half an inch (1.3 cm) from the crown. Care must be taken not to cut too close to the crown, as this could harm the plant's growing point. Removing excess leaves and stems is beneficial as it helps reduce moisture loss from the storage tissues and prevents rot. Leafy biennials like kale and collards require most of their foliage to be removed. After trimming, they should be allowed to dry for a day in a cool, shaded location. It is imperative that the main growing point, located at the top of the main stem for collards and kale, remains undisturbed during this process.
Different crops necessitate different storage methods. Root vegetables such as carrots and beets are best stored in perforated plastic bags or slatted crates to ensure adequate air circulation. These roots should be layered within containers, separated by materials like wood shavings, shredded leaves, peat moss, or moist sand to prevent direct contact and maintain proper moisture. Biennials with fibrous root systems, including kale, celery, collards, and Swiss chard, should be replanted into clean nursery containers filled with slightly moist soil, potting mix, or sand. Regardless of the storage method, plants and roots require periodic inspection throughout the storage period. Any plants exhibiting significant signs of rot or disease must be promptly discarded to prevent the spread of pathogens to healthy specimens.
Once spring arrives and the ground becomes workable, typically when temperatures are suitable, the vernalized plants can be carefully transplanted back into the garden at their desired spacing. After replanting, it is essential to water and cultivate them as needed to help them reestablish in their new environment. This reestablishment phase is crucial before the plants begin to flower and set seeds, ensuring a smooth transition and robust growth in their second year. For example, at Heritage Farm in Decorah, Iowa, rutabagas are sown in late July, allowing them to reach market maturity by October. They are then harvested and moved to a root cellar for overwintering and vernalization, demonstrating a successful practical application of these principles.
The meticulous approach to storing biennial plants is critical for maximizing seed production. By carefully managing temperature, humidity, and plant preparation, gardeners can bridge the winter gap, allowing their biennials to successfully complete their life cycle. This dedication ensures genetic continuity and supports the broader efforts of seed saving, ultimately contributing to agricultural biodiversity and resilience.