I get frequent questions about seed saving and about whether different strawberry varieties will cross. The short answer on crossing is that varieties in the same species will cross. It is not common to have crosses between different species. Hybrid garden strawberries, Fragaria x ananassa, for example will not cross with alpine strawberries, F. vesca. Hybrids have 8 sets of chromosomes (octoploid) while alpines have 2 sets (diploid).
Before leaving the question of crossing, let’s address the question of whether a gardener needs to isolate different varieties. If you are not saving seed there is no need to separate varieties of a species. Yes, they will cross but it will not affect the flavor or taste but it will affect the genetics.
All of this is important to know if you plant to save seeds. If you are growing one selection of alpine strawberries such as ‘Alexandria’ you don’t have to worry about it crossing with hybrids in your garden so you can save the seeds for the future. One thing to be aware of though, is whether there are other plants in the vicinity that will cross with an alpine. There may well be wild diploid strawberries in the area such as in the wild or those being grown by neighbors. If there are insect pollinators in the area they can cross the plants. The wind can carry pollen from distances that could cross with your single alpine variety.
If you’re getting the idea that isolation by distance may not be enough then you are catching on. Several strawberry experts have told me that plants that could cross with each other should be at least a half mile apart but to be on the safe side it should be a mile. If there are single plants or isolated patches of plants that can cross with your plants, a half mile should be sufficient. If there are large plantings within a half mile then a mile would be better. It’s all about the probability of crossing.
Are there alternatives to isolation by distance? Fortunately, yes! Isolation can be accomplished with barriers. There are many types of barriers. One might be a large planting of trees such as a forest. Mountains can isolate plantings. Even your house can provide a barrier. These barriers need to keep the wind from cross pollinating similar plants and keep bees from carrying pollen to similar plants. As you can imagine some barriers will be effective while others will not eliminate all risk of crossing.
The best form of isolation is a cage or container. We use pollination cages for our seed production. These consist of insect barrier quality netting and placement of the cages so that the wind cannot carry pollen from one cage to another. Where there might be risk of crossing we select different species for adjacent cages. We have frames that can be covered with plastic to isolate similar plants. Thought has to go into the plan for seed saving on this type of scale.
Keep in mind that plants in a cage still need to pollinate. If wind and air movement are reduced or eliminated than wind pollination will not be effective in pollinating your plants. If the plants are covered with a cage before they bloom there may be few or no insects in the cage to pollinate the flowers. We use mason bees to pollinate in our cages. One option is to hand pollinate but one must be careful when entering the cage to not let insects in. It’s a bit tricky but the main point is that there must be a well thought out plan.