Category: Garden

If you have a plant that’s a result of a cross between at least two other plants and you want to create a new stable variety from this plant, you must go through a process called dehybridization.

You can dehybridize a simple F1 hybrid variety (two plants crossed), but you can do the same thing with a grex (more than two plants crossed) as well.

I’ve mentioned in recent posts how important is is to start with the right kind of seeds when seed saving and breeding plants. Some people may be wondering why you couldn’t dehybridize a commercial F1 hybrid variety the same way you could an F1 hybrid created in your garden.

The issue is usually if you want the genes contained in a commercial variety in your garden. For example many commercial tomato varieties are bred to have tough skins or be suitable for transporting long distances. Many varieties of other plants are genetically programmed to produce their harvest all at the same time (often called ‘all at once’ types), which is handy if you are a farmer harvesting with a tractor but most gardeners would rather have a sustained harvest over a longer period. There are many other traits that exist in commercial hybrid varieties that aren’t necessarily desirable in a home garden.

If however you come across a commercial plant variety that you really want to keep and dehybridize, it’s often possible. For example, there have been a number of efforts recently to dehybridize the very popular commercial Sun Gold variety tomato. There have been a number of similar efforts made with commercial varieties in the past, and some of these efforts have been more successful than others.

The Process

As I’ve mentioned before, when two different stable varieties of plants are crossed, an F1 hybrid is created. If seeds from this generation are saved and replanted, the result is an F2 hybrid followed by an F3 and so on.

The F1 generation is distinct. That is if you grow more than one plant from this generation, they will all be the same. If you create more seeds by making a new cross from the same parent plant varieties, these seeds will also produce more of the same identical plants.

When starting with hybrid, the F2 generation the plants will start to show genetic instability, and the plants will all be different. In fact, the F2 generation will be the most unstable. It’s a good idea to grow a large number of plants in the F2 generation and save seeds only from those that have the most desirable traits, in order to take advantage of the large number of differences.

At this point it’s a matter of growing the next generation from previously saved seeds and continuing to select the most desirable plants. If you come across a generation without plants with desirable traits, it can sometimes be necessary to replant the previous generation, so it’s a good idea to save seeds from earlier plantings. If you are dehybridizing an inbreeding plant and end up with more than one plant with different desirable traits, it may be necessary to cross pollinate these plants so the desirable traits are combined. If you are working with outbreeding plants they will probably be cross-pollinating on their own anyway, making good rouging practices very important.

If you decide along the way new genetic material is needed, you can also introduce the genetics of other plants by cross-pollinating with them. Consider that the more complex and varied you make the genetics, the longer it will take to dehybridize and stabilize the variety.

After a number of generations of saving seeds from desirable plants, and rouging undesirable plants, the genetics of the new variety will begin to stabilize and you will seed fewer differences between the plants. Eventually, you will have a new stable open pollinated variety. Different plants take longer or shorter times to stabilize, ranging from a couple of years to as many as 10-15.

Like my other recent seed saving posts this one is mainly targeted at vegetables, but some of the ideas here may be able to be applied to other plants as well. I will also be trying to address the simplest ideas of this subject, and for those who want more detailed information I would refer you to the two books I mentioned in my last post.

Many people who save their own seeds avoid the issue of cross-pollination by only growing inbreeding plants. When growing out-breeding plants, or breeding your own varieties, it’s important to know which plants in your garden will cross with each other.

Unfortunately, this is not an exact science. Sometimes plants that are not supposed to cross do, and others you think should cross don’t. Sometimes vegetables can unexpectedly cross with nearby flowers. Opinions change on this subject, and reference books are rewritten. Often gardeners and/or scientists have conflicting opinions on this subject. There is however a lot known about this subject, and when you are thinking about saving seeds from an out-breeding variety it’s worth researching it and seeing what you can find.

The first thing you should look at when considering the possibility of plants crossing is the Latin names of the plants involved, and the families the plants belong to. I wrote a post about this several months ago. Usually, but not always, plants with different Latin names will not cross with each other. Most plants with the same Latin name will cross with one another.

Besides reference books, and the Latin names, your best bet is to ask other gardeners what their experiences are. In addition, there are a few commonly known guidelines for some vegetables, and I’m going to cover some of them below.

Squashes and Pumpkins

This type of plant has four common different species, and therefore different Latin names: Cucurbita pepo, Cucurbita moschata, Cucurbita maxima and Cucurbita mixta. These don’t usually cross with each other, and by choosing carefully it’s possible to grow four different varieties next to each other without cross-pollination issues.

Many people employ hand pollination techniques to keep different varieties of these plants from crossing, but I’m not going to get into this here.

Melons

Almost all melons are Cucumis melo, and will cross-pollinate with one another.

Beets and Swiss Chard

All beets (garden, sugar and mangel) and Swiss Chard are Beta vulgaris, and will cross with one another. These are also all biennials, and will go to seed in the second garden season. These are out-breeding, and the pollen is carried by the wind.

Beans

Most ‘ordinary’ beans are Phaseolus Vulgaris, and will cross with one another. These are mostly inbreeding plants, but are sometimes cross-pollinated by insects. These beans will not cross with other kinds of beans.

Lima (butter) beans are Phaseolus lunatus, and are somewhat out-breeding plants that will cross with one another. They are usually crossed by insects, so different varieties can be grown next to each other if they are grown under row covers or other means are used to keep pollinating insects off of them.

Soybeans are Glycine max, and are mostly inbreeding so different varieties can usually be grown next to each other.

Carrots

Carrots are Daucus carota, and are very outbreeding plants. The pollen is very fine and carried long distances by the wind, making it very hard to save seeds from more than one variety at a time. To make matters worse, carrots will cross pollinate with Queen Anne’s Lace, a common weed in many areas. In fact if Queen Anne’s Lace is very common in your area, you may simply not be able to save pure enough carrot seeds.

Because Queen Anne’s Lace has a white root, if you are saving seeds from white carrots you may not notice crosses when they do occur.

Because carrots are a strongly out-breeding plant, a minimum of 40-50 plants are generally needed to insure a large enough genepool remains in the saved seeds, but these can generally be planted very close together to conserve space in the garden.

Brassicas

In English, these are often called cole plants, and are the cabbage family.

Cabbage, Broccoli, Brussel Sprouts and Cauliflower are Brassica oleracea, and will easily cross with one another. Most other Brassicas are distinct species and will not cross, but sometimes they do! Turnips and rutabagas are distinct species and will not cross with each other or Brassica oleraceas.

These are very out-breeding plants, and depend on insects for pollination. Because plants are generally not able to self-pollinate, but rather require an insect carry the pollen from one plant to another, isolation is not as simple as covering the plants with row covers. In fact, if you normally cover these plants with row covers in order to keep caterpillars off of them, you will need to remove these to allow cross-pollination of the plants.

Isolation of the plants can be achieved with covers on alternating days, usually this is done with a screen cage that can easily be picked up and moved.

Because these are very out-breeding plants, often a large number of them are needed to maintain adequate genepool size, often requiring a lot of garden space.

Because of all these complexities, Brassicas are often thought of as more advanced plants for seed saving.

Corn

Corn is very out-breeding, and the pollen is carried long distances in the wind. If you live in an area where corn is grown commercially, there may be so much pollen in the air that it’s impossible to properly isolate your plants from cross pollination. This is particularly important of course if you are trying to protect your seeds from contamination from locally grown commercial GM varieties.

Bagging and hand pollination techniques can be used to grow different varieties next to each other, but I’m not going to get into this here.

It’s also possible to choose an early and a late variety, with different pollination periods, and collect seeds from both without cross-pollination problems.

Because corn is very out-breeding, you will need 150+ plants in order to maintain the genepool.

There are two ways the genetics of the plants in your garden can change, with sports or crosses.

Sports

A sport is a chance genetic variation. Søren recently came across a sport in some seeds I sent him. Sports can be good or bad. Many very interesting plant varieties are the result of sports. Plants that result from a sport are still genetically stable, and preserving the new trait is as simple as saving seeds from it. It may take a few generations of selective seed saving before the sport itself becomes a stable trait.

Plants that are inbreeding or don’t develop true seeds which have resulted from pollination (like garlic), depend entirely on sports for genetic evolution. For this reason, many of these plants are more genetically prone to sports.

Crosses

Crosses can be accidental or intentional.

Crosses are when two genetically compatible plants of different varieties exchange pollen with one another, resulting in an F1 hybrid. F1 hybrids are distinct in the first generation, but genetically unstable in future generations. In this case the resulting plant has 50% of it’s genes from each parent plant. Accidental crosses are often obvious in the garden, especially when it involves 2 very distinct parent plants. When growing 2 similar varieties next to each other, a cross may not be immediately obvious and this is something to consider when selecting varieties to be grown in close proximity.

Gregor Mendel is credited for being the first to document what happens when plants cross, and the consequences of dominant and recessive traits. A while ago, I wrote a post about this.

In order to create a new, genetically stable, variety from an F1 hybrid you must go through a process called dehybridization. This is an obvious candidate topic for a future post.

This is where seed saving can become very difficult. Like the seed saving posts from the last few days, I’m going to focus here on the simplest aspects of out-breeding plants. Let’s also be very clear here, I have about 5 years experience with seed saving. For some people that may make it seem like I am an expert, but many people spend their whole lives working in this field and are still learning new things. In fact anyone who tells you they don’t have anything new to learn about seed saving is probably lying to you. Even well known published books on the subject come out with new editions and changed information, as opinions change and new information is uncovered.

Think of this post as something to get you started thinking on this subject, not a how-to guide! Also remember everything I say here is a guideline, not a rule. The great thing about seed saving is you can do whatever you want! Your plants belong to you, and the whole point is that you should be growing the plants you want to. If ignoring something I or anyone else says on this subject means you grow something more interesting, it’s a good thing. There is also nothing lost by experimenting. Just be sure to be honest with anyone you share your seeds with about what they should expect!

Be sure to start with the right kind of seeds!

New Page

Because there are now many people reading this blog, with many different backgrounds and skill levels on this subject, I’ve created a new Common Terms page that appears on the front of this blog. This page has some definitions for common terms and concepts related to seed saving and plant breeding, and I’ll be regularly adding new things to it. As an alternative to explaining these concepts every time I post about them, I will link to this page instead.

Good Books on the Subject

If you want to read more and probably more accurate information on this subject, these are two good books:

Carol Deppe’s Breed Your Own Vegetable Varieties: The Gardener’s & Farmer’s Guide to Plant Breeding & Seed Saving

Suzanne Ashworth’s Seed to Seed: Seed Saving and Growing Techniques for Vegetable Gardeners

The Issues

There are three main issues with out-breeding plants, inbreeding depression, rouging and cross-pollination.

To avoid inbreeding depression you need to save seeds from enough different plants of the same variety at the same time.

To avoid unwanted traits from entering your plants genepool you need to rouge out or remove plants with these traits.

In order to avoid unwanted crosses in your garden, you need to make sure your plants are isolated from other genetically compatible plants.

At the Simplest Level

If you have an out-breeding plant you want to save seeds from, just take into account these three issues and nothing more is required.

The number of plants needed to avoid inbreeding depression varies, but is generally between 5-200. Obviously more is better, but 10-20 is a reasonable number for most plants. If you can manage 200 plants, you are virtually assured to avoid the problem. In any case, problems with inbreeding depression will not show up for a few years, so you are very likely to end up with at least a few generations of plants regardless of how many plants you manage to save seeds from. If you know your genepool is limited, and inbreeding depression in future generations is likely, don’t share your seeds without warning the next person!

Be sure to rouge out plants that bolt early, are stunted or in any other way deficient. When considering inbreeding depression, and how many plants you will need to avoid this, remember to plant some extras so some can be rouged out and you will still end up with enough plants.

You don’t need to worry about very unrelated plants crossing, for example a cucumber will not cross with a watermelon, but otherwise make sure there are no similar plants blooming anywhere in the area at the same time yours are.

That’s it!

Some out-breeding annuals to try in this way are pumpkins and squashes, cucumbers, some herbs like basil, sunflowers, fava (broad) beans, lima (butter) beans, melons and spinach.

To avoid vitality problems with the seeds, be sure not to eat from the same plants you save seeds from. The plants should put all of their energy into the seeds.

Where Does it Get Difficult?

The problem is most people want to save seeds from more than one variety at a time, or cannot be sure their garden is properly isolated from other genetically compatible plants. Also, determining exactly what is genetically compatible or the true isolation distance required can be very difficult, and often requires some trial and error. In cases where sufficient isolation distances can’t be maintained, you need to use techniques like caging, bagging or hand pollination. Rouging can also be more difficult for example if assessing a plant for deficiencies involves a root crop buried in the ground, or a vegetable that forms after cross pollination takes place.

Biennials can also be considerably more work than annuals. Among the issues to be dealt with, they go to seed their second growing year requiring more time and space in the garden and often need special care to survive the winter in colder climates. Biennials also often require larger genepools to avoid inbreeding depression.

Perhaps in some future posts I will address some of these issues, and discuss seed saving from specific plants.