Isolation refers to the techniques of protecting the varietal ‘purity’ of seeds by ensuring that plants can’t cross-pollinate or genetically mix during the flowering stage (before they set seed). Different plants (varieties) of the same family may cross-pollinate when pollen is transferred from one plant to another by wind, insects or human intervention. By isolating different plants of the same family from each other, cross-pollination can be prevented by ensuring that pollen cannot travel between plants.
Isolation can be achieved through distance, time or physical exclusion. Read on to learn more about these isolation techniques which prevent cross-pollination.
Only plants which are ‘outbreeders’ or which do not have self-fertile flowers need to be protected against cross-pollination. Plants that are self-pollinating and therefore don’t cross-pollinate (or have extremely low rates of cross-pollination) include lettuces, tomatoes, peas and beans. You may still want to isolate these plants to be 100% sure of varietal purity but it less likely to be a concern.
Refer to the Guide for each plant family to check whether plants of that family are likely to cross-pollinate and to identify their pollination method (wind or insect or both) and then check out then read on to learn more about the appropriate techniques of isolation.
Because it may be impossible to sufficiently isolate plants for varietal purity in urban or suburban settings, hand pollination is sometimes used to control the transfer of pollen between flowers. To learn more about pollination including hand-pollination, cross-pollination and hybridity, see the Pollination section.
To stop pollen being blown from plant to plant by the wind; or carried by insects as they fly between plants, plants of different varieties from the same family can be isolated by distance.
Isolating by distance means keeping plants of different varieties from the same family far enough apart from each other that either:
- the pollen blown on the wind from one plant can’t reach the flowers of another; or
- pollinating insects cannot fly the distances between plants of the same family.
The distances required between plants will differ depending on the type of plant and its method of pollination. Plants pollinated by insects, such as pumpkins, need to be separated by large distances – bees can fly up 3 or 4km depending on conditions! Plants pollinated by wind, such as corn, need to be separated by enough distance that pollen will not carry on the wind from one plant to another. This will depend on the pollen, position of plant in a garden and any physical barriers between plants.
Check the plant guides for recommended isolation distances for plants.
Isolation in time means not letting plants of the same variety from the same family flower at the same time.
For plants that flower continuously or for long periods over a season, this will most likely mean only growing one variety from a family in each season (note, this doesn’t apply to self-pollinating plants like tomatoes, peas, beans and lettuce).
For plants that produce a lot of flowers that are pollinated and set fruit or seed in a very short period of time, it may be possible to time planting and flowering of more than one variety in the same family so that they don’t flower at the same time. This might mean letting one flower early in a season and only letting another variety later in the season when the first variety has stopped flowering.
This techniques is easier in places with longer growing seasons.
Physical exclusion means creating a physical barrier that prevents pollen being transferred between plants. This is easiest to do with plants that are insect-pollinated. Effectively, you find physical ways to keep the insects away from the pollen and do the pollination yourself (‘hand-pollination’ or ‘being the bee’). It is possible to physically exclude cross-pollination with plants that are pollinated by wind but it requires good planning at the beginning of the season about where plants are grown and what structures and other plants are placed in your garden.
Techniques for physical exclusion of insects can include bagging flowers before they open, caging entire plants, erecting artificial barriers, growing other plants as living barriers, and growing plants separated by large structures like houses or sheds.
These techniques will work with varying degrees of success depending on the type of insect pollinator and prevailing conditions. The only certain methods are bagging flowers before they open and hand-pollinating, or caging entire plants and hand-pollinating.
For plants that are wind-pollinated, bagging flowers or caging plants is not appropriate because it prevents the wind carrying pollen between plants of the same variety and stops any pollination happening (meaning no food or seeds!). The most effective physical exclusions for wind pollinated plants include erecting artificial barriers, growing other plants as living barriers, and growing plants separated by large structures like houses or sheds. It is important that these barriers stop any wind travelling between plants of different varieties from the same family but allow plants of the same variety to pollinate each other.
To learn more about hand-pollination which is required to ensure insect-pollinated plants which have been bagged or caged will still produce fruit or seeds, see the section on Pollination.