Mechanical isolation involves the use of physical barriers to prevent any contamination from foreign pollen. Such barriers include fine meshes, screened tunnels and cages, bags, thickly planted shrubs or trees. Each solution has their own advantages and disadvantages. While vegetative barriers only partially block unwanted pollen carried by wind and insects, other solutions such as cages and bags offer more control and will efficiently reduce any risk of crossing with nearby varieties. However, those methods usually require more work, time and material per plant population. In some cases, a manual introduction of pollinators inside the cages or tunnels as well as alternate day caging techniques are necessary to ensure seed purity.
Bagging is one of the easiest isolation techniques to implement, although potentially more time-consuming depending on the size of your plant population. It is particularly appropriate to prevent cross-pollination of self-pollinating plants. It involves the covering of the flowering portion (individuals or clusters) of a plant with a mesh bag in order to isolate the flowers from pollen carried by insects. The bag should be securely tightened around the stem in order to prevent pollinators from entering. When the flower has withered, the bag is removed so the vegetable can develop undisturbed. The fruit that is being reserved for seeds is then labelled. Note that only flowers that are still unopened should be bagged, as open ones might already have been pollinated by insects (and thus contaminated).
The isolation of wind-pollinated plants such as corn (specifically the tassels and ears) will require the use of bags made of waterproof paper, which will prevent the plants from being contaminated by foreign pollen grains from undesired sources.
Through the use of screened cages or tunnels, caging techniques are very versatile and efficient solutions in the isolation of crop varieties grown for seed production. While they usually require more material and maintenance, screened cages and tunnels can be built inexpensively by using wood, rebar, poles of several kinds, nylon or fine insect netting. Cages and tunnels should be built high and wide enough in order to accommodate the plants’ mature size and inflorescence. Both plants with perfect flowers and self-fertilizing crops can be grouped together under the same cage or tunnel to optimize space. When appropriate, whole plants can also be wrapped individually by using insect netting and a few poles, which are then secured at the base with clips or rocks. Note that while such structures must still allow sufficient air and light to pass through while excluding insects, the netting used will inevitably block a small portion of the sunlight, which might be an issue in areas with short growing seasons.
Alternate day caging can be used when two or more varieties of the same species are grown in the garden and cross-pollination must be avoided. Those can be grown under separate screened tunnels, each opened and closed alternatively (every other day) at flowering time. The tunnels should be open early in the morning and closed late in the afternoon to benefit from the presence of pollinators and increase the likelihood of successful pollination for each variety. Alternate day caging will, however, gradually limit seed overall production as the number of cages or tunnels increases. The more cages in rotation, the harder their management, the fewer flowers are subject to be pollinated on a daily basis, which will result in a reduced seed set. To compensate, plants might occasionally produce flowers over a longer period of time. Also note that this method still does not eliminate the risk of cross-pollination if other varieties are flowering in neighboring gardens. This must be taken into consideration along with your environmental conditions when such technique is implemented.
To increase seed production and ensure seed purity, the cages and tunnels should only be removed at the end of all plants’ flowering period and/or when the seeds have begun to dry. This process can, however, be shortened whenever the desired number of fruits/seedpods has been reached.
Introduction of pollinators
In cases where allogamous plants are to be isolated, pollinators such as flies or bees will need to be introduced. If so, only one variety with several individuals should be grown per cage or tunnel. The cages must be large enough to both cover the plants and allow the insects to fly freely in order to optimize the transfer of pollen between flowers. While flies and bees can be captured and placed inside the cages, this solution is often ineffective (or yields low results) if not detrimental, as trapped bees might carry unwanted pollen from other species. The pollination will likely be poor, with bees being distracted and trying to escape from the cage. To increase chances of successful pollination, smaller beehives such as nucs filled with ready-to-hatch bees can be introduced. Having no prior pollination work experience, those will more likely feed and pollinate the flowers located in their immediate surroundings. Note that the introduction of pollinators remains a costly solution (beehive rental) and might require more experience as it is not commonly used by home gardeners.