Vernalization and Overwintering


Vernalization is the induction of a plant’s flowering process by exposure to the prolonged cold of winter, or by an artificial equivalent. Many perennials, monocarpic winter annuals and biennials must go through a prolonged period of cold before flowering occurs. A biennial crop will indeed only flower and set seed in the second season provided the three following criteria are met: the plant must go through a period of vernalization (a period of low temperature); the plant must be appropriately sized during its first growing season; the plant must survive the winter (in the relevant regions). The period of cold (often expressed in chill hours) needed by biennials grown in temperate climates varies from a crop to another, but exposure to temperatures ranging from 0° C to 5° C for at least 10 weeks on average is usually enough for most vegetable crops. After this vernalization process, plants will acquire the ability to flower (and set seed) but may require additional seasonal cues or weeks to do so.

Biennial plants can overwinter either directly in the field in mild temperate climates (referred to as the seed-to-seed method) or in an ex situ controlled environment using the (seed-to)root-to-seed method – often used in areas experiencing cold winters. In this case, the crop’s root/bulb is harvested (known as stecklings), stored away to overwinter in a root cellar or cold room, then replanted the following year to produce seeds. While the former method is easier to implement, the latter will have the benefit of allowing a better plant selection as the physical characteristics and other traits of the roots/bulbs can be easily assessed. Overwintering requirements will depend on climatic conditions, the biennial plant type (root or non-root), and will vary from one species/variety to another. Knowing your plants’ physiological pattern is important, as it will allow you to identify the optimal overwintering method, storage environment, and to carefully select the best individuals to produce seeds.

In Situ Overwintering

In areas where winters are mild (but cold enough to allow sufficient vernalization) and risk of frost damage is absent, seed crops are commonly left in the field to overwinter under ambient environmental conditions without – in most cases – requiring any extra work. Some biennials can tolerate lower temperatures than required depending on the variety or if the crop has been adequately and slowly hardened off to the cold. Some varieties of Brussels sprouts and parsnips are indeed known to be cold tolerant and to resist temperatures of at least -12° C.

A good knowledge of the region and its climate is important as many temperate areas are subject to days of extreme cold during peak winter weather, which can significantly impede the overwintering of the crops outdoor. Using a thick layer of mulch will help insulate the plants. Depending on the scale of your production, greenhouses or cold frames may appear as more suitable options. Root crops that have been dug up and selected can be used in conjunction with pitting, a traditional technique which involves storing the roots in a pit dug below the frost line and backfilled with soil. Alternatively, in larger seed production operations, the roots can be dug, topped, windrowed, then covered with soil to prevent freeze damage for that specific climate, which serves as insulation.

Ex Situ Overwintering

Following the vegetative growth phase of the crop, the best roots that are to be overwintered ex situ are selected based on important traits that characterize the variety that is produced, such as the shape, size, color, flavor, texture (by carefully “cheeking” the root if dense enough), branching tendency (often caused by soils with high clay content), disease resistance and exterior smoothness when applicable. This step is crucial to help to maintain uniform root-quality characteristics as well as genetic purity. While easy, preparing the stecklings for storage still require dexterous handwork. The leaf tops must be trimmed at about 1.5 cm above the crown, making sure not to damage the apical growing point. Then approximately 8 to 10 cm of the taproot is cut, as well as any petioles which might lead the root to rot. The stecklings are then carefully cleaned to remove any soil without using any water (by rubbing or shaking) as it might promote rot when stored and remove potential beneficial bacteria located on the outer layer.

With few exceptions such as onions, the long-term storage environment must meet the following three criteria that are to be imperatively maintained: a suitable cold temperature range (1 to 3° C), a high relative humidity (RH) of 95% or higher, and good airflow. Storing stecklings at higher temperatures will promote their respiration, using more of their carbohydrate reserve, and will prematurely put on both root and apical shoot growth, often long before the appropriate spring weather for replanting in the field has arrived.

Note: Due to their large vegetative parts that can easily rot, non-root biennials are usually not well suited to storage, rarely overwintered in cold rooms, and are therefore best stored with their roots in the soil.

The stecklings can be either stored in a root cellar in containers/bins or a cold room on shelves. When stored in containers, it is preferable to lay the roots between layers of clean, slightly damp sand, newspaper, or undecayed deciduous leaves, making sure they do not touch as it would increase any emergence and spread of rot. When a cold room is used, the roots can simply be stored on shelves or shallow flat containers covered with clean wood chips. This will keep the roots free of standing water that can accumulate on the surface of the uppermost roots under high-humidity conditions, therefore helping to eliminate any potential source of rot. In storage environments with lower-than-optimum RH, the roots can be bagged in plastic bags with air holes to increase and safely maintain the relative humidity while allowing the excess moisture to escape. As there is still a risk that moisture condenses and remains in the bags, it is recommended to add a few handfuls of wood chips, spread evenly among the roots.

The stecklings should ideally be checked every two or three weeks. Any root exhibiting rot damage should be eliminated at once. In the following year (reproductive season), the roots are evaluated and the best ones are selected for replanting.

Specific overwintering practices for each applicable crop are discussed in Module VII.