To ensure the seed we save produces what we expect it to we first need to understand the reproductive method used by the variety we’re planning to save seed from.
[More detail on reproductive methods is here]
If we are growing a self-pollinating variety then we generally do not need to do anything else to ensure our seed will be true-to-type. Easy!
If we are growing a cross-pollinating variety then we may need to take some action to ensure that the variety we want to save seed from does not cross with another variety of the same species.
Actions we can take include:
- isolating by distance – growing far enough away from other varieties so that insects or wind are unable to transfer pollen between the varieties;
- isolating in time – only allowing one variety to flower at a time;
- physical isolation – use barriers such as cages, nets and bags to physically stop the transfer of pollen between varieties.
Isolating by Distance
The isolation distance is how far we need to grow a variety from others it might cross with so that we can be reasonably sure that the seed we collect will grow true-to-type and produce what we expect.
Why are there different isolation distances for different plant varieties?
There are different isolation distances specified depending on whether a plant is a self-pollinator, is cross-pollinated by insects or cross-pollinated by wind (or sometimes a mix in which case we use the biggest distance).
The isolation distances suggested for self-pollinating varieties is mainly so that we don’t confuse the crops during harvesting – which is easier to do than you think if they are right next to each other and are all brown and dying back with mature seed on them. It also ensures enough distance so that your taller crops can’t fall over in the wind into the next one and so that any climbers can’t travel along and hide among a neighbouring seed crop over the growing season. If you are tempted to use one end of a trellis for one type of bean and the other end for a different one then make sure you pick varieties with distinctly different bean seed colours and patterns.
Also, the closer you plant different varieties, the more you risk an insect that can cut through the flower to get at the pollen and nectar walking over to the next plant to do the same thing and making an unplanned cross.
If you are in Tasmania you need to be aware of bumblebees that are renown for cutting their way into self-pollinating flowers. The rest of Australia doesn’t have this particular insect to worry about.
Cross-pollinated by insects
For most of Australia the main insect pollinators are honeybees supported by native bees, wasps, flies and some ants. So for plants that are pollinated by insects the distances are based on the foraging behaviour of bees.
While bees can forage many kilometres from their hives they tend to find and then focus on a particular crop or plant that is producing good pollen and/or nectar at that time. So, for example, they’ll pollinate all the pumpkins in the area they are working in before moving on either to another crop or another location.
The isolation distance for cross-pollinating by insects makes some assumptions about how far they have to travel to find food and what obstacles are in their way. Buildings and wind-breaks can have an effect on where bees forage. Also having lots of other flowering plants to distract the bees means they’ll stop off on the way, probably dropping pollen they’ve just collected and picking up something different instead.
Cross-pollinated by wind
Plants that are pollinated by wind typically have very fine, light pollen so that it easily travels on the breeze. This very light pollen can be carried surprising distances, so the isolation distances given for wind pollinated varieties are usually very large.
Usually isolation distances are suggested based on worst case assumptions about the terrain and prevailing wind direction. For example, if you are growing on open, flat land and are down-wind from a flowering crop of a variety that your seed crop might cross with then you’d want to have a very large separation distance. If the potential crossing crop is behind some buildings and lots of trees and you never get a breeze from that direction during the flowering season then you could get away with a much closer distance. Local knowledge is key. If you don’t know then using the suggested distances is usually the safest option.
Why are Isolation Distances sometimes given as a range?
As discussed, the specified range of isolation distances is partly because they are trying to allow wiggle room for different local growing conditions. But it is also about how much risk you are prepared to take that things might cross.
If you are just saving a bit of seed for yourself and it is a variety that is quite common and you can easily purchase replacements from lots of seed suppliers then you might not be very concerned if a cross did occur. You just want to have some reasonable assurance that you aren’t completely wasting your time. In that case you could use the smallest suggested distance.
If you are the custodian of something really rare that nobody else has and it might be irretrievably lost if a cross occurs then you would want to take extra care and would probably use the biggest suggested distance in the range.
Why don’t isolation distances always work?
Nature isn’t as black and white as we’d like to believe. The given isolation distances are the best practical advice to cover most situations. Sometimes unusual circumstances occur. For example in some situations up to 5% of lettuce flowers might not fully pollinate before opening. This isn’t usually a problem because no insect is going to waste all their energy searching through all the flowers for the small number they’ll find food in. They’ll go somewhere more productive. So generally lettuce isn’t very attractive to insects. But sometimes you’ll get an unexpected cross in your lettuce patch. That isn’t a problem, just eat that one and save seed from the remaining ones to keep your line true-to-type. Or you can save seed from it and enjoy some new lettuce types. If you regularly have a problem then understanding your conditions, the terrain, the local pollinators and the pollination mechanism used by that variety can help you work out how to solve the problem.
Isolating in Time
Some crops, like corn, produce pollen over a just a matter of weeks rather than continuously over the entire growing season. This means that crops of this type can be planted with enough time between each planting such that both do not produce pollen at the same time. This allows us to grow several crops of the same variety in the same season without the risk of them crossing.
To ensure that cross pollination cannot occur physical barriers such as fine insect mesh can be used to ensure that pollen cannot be transferred between plants. Flowering plants can be caged or bags placed over flower heads to stop pollen being transferred from other plants. Crops can be grown in screened enclosures, such as mesh tunnels, either with all pollinators excluded or with a hive or insects introduced into the enclosure when pollination is required. Alternatively, if controlled cross-pollination is required it can be carried out by hand.
Contributors: Liz Worth, Julie Davies