Plant Lifecycle, Plant Needs, and Photoperiodism

Plant Lifecycle

Knowing the life cycle of the plants that you intend to grow for seeds – and how long it will take them to reach maturity – is critical to ensure the success of their production. The two main types of crops we usually save seeds from are annuals and biennials. An annual crop only requires one growing season to produce seeds and complete its lifecycle. Tomatoes, beans, corn, and squashes are such crops. Biennial plants require two growing seasons and will produce seeds and complete its lifecycle in the following year. Examples of biennials include carrots, beets, and cabbage. Most of the biennial crops that are usually eaten for consumption are actually grown as annuals, never completing their full lifecycle.

A biennial crop will 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.

Plants that are too small in autumn might not reach their full size before flowering or respond to vernalization. This will result in the absence of flowers, making seed production impossible. Some might lack the hardiness required to survive in the field and withstand the cold temperatures of winter. They will therefore need to be dug up and stored for several months in a cool but not freezing location in order to overwinter. Plants that overwinter this way are then replanted the following spring to complete their lifecycle.

Specific overwintering instructions are discussed in Section VII for each biennial crop.

Plant Needs

Learning each seed crop’s history in cultivation, evolutionary past, lifecycle, climate, as well as day-length requirements is critical to help you understand and find the optimal cultivation environment in which the crop will thrive. Understanding each crop’s needs in the context of seed saving is equally important, as some specificities will apply individually. Since plants for seed production will remain in the field longer than crops for consumption, an adequate rich organic soil slowly feeding the plants through the entirety of their lifecycle will be necessary. Phosphorus is notably a critical element, as it contributes to the growth of flowers and seeds. It is naturally found in certain manures, bone meal, and pulverized rock phosphate, which will maximize the reproductive capacity and subsequent seed yield of flowering crops. A good balanced source of nitrogen is also needed. A lack of nitrogen usually results in stunted plants and lower seed yields, while an excess will lead to an increase in vegetative growth, which can make plants more prone to lodging and might leave them more exposed to diseases and other insect attacks. Learning the common plant health indicators will further help you identify any potential nutrient deficiency.

Specific tips and instructions on how to build and nourish your soil organically are given in Section III.

Spacing is also a parameter to take into consideration when planting, as crops intended for seed production will grow taller and wider as they reach full maturity than the ones grow for regular consumption. The planting is done similarly, but crops are thinning out with seed production in mind, resulting in a plant population spaced further apart.

Certain plants require a certain amount of warm days to develop well and complete maturity. Growers located in regions with short or cool growing seasons might need to conduct experiments to see which plants can produce seeds successfully based on the specificities of their climate. Some environmental conditions could also make the pollination of certain crops difficult and will require careful observation as well as further research.


Photoperiodism is the physiological reaction or developmental responses of a plant to the length of daylight and darkness periods it experiences. Plants are classified in three categories: long-day plants, short-day plants, and day-neutral plants. Because photoperiodism is a determining factor in the reproductive growth of some plants, an understanding of this process in each of them allows a better planning of your garden in order to ensure the production of seeds.

Long-day plants are plants that require more than 12 hours of sunlight, or less than 12 hours of uninterrupted darkness, in order to produce a flower. They typically flower during late spring or early summer, as the days are getting longer. Common examples of long-day plants are beets, carrots, fennel, lettuces, potatoes, radishes, spinach, Swiss chard, and turnips.

Short-day plants are plants that require less than 12 hours of sunlight, or more than 12 hours of uninterrupted darkness, in order to produce a flower. They typically flower as days grow shorter (and nights grow longer) during summer or fall. Common examples of short-day plants are soybeans, beans, cotton, and rice.

Day-neutral plants are plants that flower regardless of photoperiods. Most of them initiate flowering after a specific development stage or age, or in response to environmental factors and stimuli, such as vernalization. Common examples of day-neutral plants are cabbages, corn, cucumbers, tomatoes, peas, and sunflowers.