On Inbreeding Depression



This Acorn Squash is a good example of an outbreeder

In order to remain self-sufficient, many gardeners choose to save seed from their favorite fruit and vegetable varieties. When seeking to save seed properly, it is important to know what you are doing. A seed saver needs to know how to grow a plant, how the plant pollinates, how to save seed, and other factors that may affect future seed viability including plant selection, variety isolation and inbreeding depression.

I began researching inbreeding depression in order to help me better determine how to plant my garden in the future. I recently read about it in The Heirloom Life Gardener by Baker Creek seeds and in The Resilient Gardener by Carol Deppe. Inbreeding depression occurs when plants have too small of a population of fellow breeders during pollination. It causes future generations of plants to be weak as demonstrated by slow growth, poor vigor and disease resistance, along with poor fruit production. Some varieties of plants such as tomatoes, which can self-pollinate, have a much smaller problem with inbreeding depression than plants such as corn, that require a population of at least 200 plants to keep from inbreeding depression.

In The Resilient Gardener, Carol Deppe says the following about inbreeding depression for corn: “Corn is very subject to inbreeding depression. Here’s an illustration: Suppose you grow up a hundred plants and save grain each year from the plant that yields best. What kind of yield will you have in five years? Answer: Your corn variety will be so weak, wimpy, and low-yielding that you will be lucky to be able to keep it going at all. Its yield will stink compared to what you have started with, even though you selected for the highest yield each year.” pg. 282

Carol Deppe goes on to explain some standards to live by to keep from inbreeding: “The standard seed-saving rule of thumb is to save seeds from 20 plants if the species is an inbreeder and from 100 or more plants if it is an outbreeder”. Pg. 304

So what is an inbreeder? An inbreeder is a kind of plant that can pollinate its own flower if needed. Each flower contains all parts needed for pollination and fruiting. Tomatoes, peppers, eggplant, peas, and beans are good examples of inbreeders. Though cross-pollination may not happen too often, it is important to grow a large enough population on inbreeders that if a detrimental trait arises you can select out the bad plants and keep only the best plants out of the population.

An outbreeder is a plant that has separate pollen-producing and pollen-receiving structures within the plant. Outbreeders include the cucurbit family, many grains, melons, and most other garden plants.

Each vegetable variety has various tolerance to inbreeding depression but it is good to follow the 20/100 rule to keep from having long-run problems with your seed. That is with exception of the maize family, which requires 200 plants to keep from exhibiting inbreeding depression.

Having to select the best plants from each population and having to make sure there is a large enough population to keep from inbreeding depression can be reason to cause depression among many home gardeners. So what can a gardener do to make sure that inbreeding depression does not happen? A gardener could make sure that he grows enough of a crop each year, or he could save seed from multiple years while using different seed each year, or he could grow the same variety with other gardeners and swap seeds to keep genetic diversity high. The last option assumes that both gardeners are selecting seed for attributes that both have determined are the most important. Another helpful practice for gardeners saving seed would be to mark what year or batch the seed was grown in and note how many plants were in the population that produced the seed.

Some other websites that are really helpful in understanding inbreeding depression include Seed Saver's Exchange Seed Saving Chart. For seed saving, gardeners should at least grow enough plants to maintain a variety. There is also a website that tells about those crops that are most tolerant and sensitive to inbreeding depression.

My current winning tomato variety - Siletz



Siletz tomatoes that lack pollination also lack seeds

This year all the tomato plants in my temporary greenhouse are from the variety Siletz. I usually diversify in the varieties I plant but I was so impressed with the performance of this cultivar last season that I only planted seeds of this variety. I'm still planning on starting a couple other varieties soon. Siletz was the only variety that could even compare to Celebrity in production.  And since the taste of the tomato fruit is the least of concerns for Tucson’s tomato gardeners I am willing to have food production be pretty high on my priority list. Don’t get me wrong – a tomato’s taste is important but without shadecloth most tomatoes are lucky to produce a few fruit in Arizona’s summer heat. Many varieties that are advertized as producing an abundant crop of large tomatoes just barely present 1 or 2 cherry-sized tomatoes per plant. When planting anything in my garden I first prioritize what I want. The three things that matter most to me are that the plant is open-pollinated, that it can grow in my climate, and that it produces lots of food. Growing in my climate requires that the plant exhibit vigor, disease resistance, heat resistance, and large fluctuations in temperature.

Although some may scoff at the taste of a Celebrity Hybrid tomato, none will laugh when you compare a tomato plant with 25-50 large fruit on the vine with what many heirloom plants produce in my climate - almost nothing. For this reason some gardeners who try Celebrity may choose not to go back to heirloom tomatoes. This is also the reason that some gardeners who do not grow hybrids make an exception for Celebrity. So why have I been growing a hybrid tomato in my heirloom garden? Because I use Celebrity as a standard to compare my other tomatoes to rather than as a long term solution to my food production needs. It is unfortunate that, within my garden, Slietz has been the only other cultivar that has even compared to Celebrity in fruit production. Both varieties possess a semi-determinate growth habit. Siletz's parthenocarpic nature also allows it to pump out fruit in the middle of the summer when most other varieties begin waning. This last year I planted most of my tomato varieties in a trench (lowered bed gardening) in the middle of my yard and provided west side shade from a bean trellis. The lowered bed method is also used by the Community Gardens of Tucson and is very useful for southwestern gardening. My seed source for this Siletz was Territorial Seed Company.



End of Season Roots of Siletz, Celebrity, and Ozark Pink Tomatoes.

Last crop of the season ready for paper bags. No other varieties compared.

Tomato Growth Patterns

It seems that some of the best tomato varieties I have trialed here in Tucson are those that continue to grow new branches despite having a shorter growth pattern. These tomatoes are often called semi-determinate because they exhibit the growth pattern of an indeterminate without having the size of one.



The wild tomato variety, L. hirsutum exhibits growth size based on pot size.

Determinate tomatoes usually grow short bush-type plants and have a crop that ripens all at once. This is very helpful with a farmer who harvests all his nearly ripe green tomatoes with a tractor. Meanwhile, indeterminate tomatoes are characterized by tall growing tomato plants that can often grow as tall as a small tree and often require support to keep them from rambling. Determinates tend to do very well in pots while indeterminates tend reach their full potential when grown in the ground. Gardenweb has a good explanation of both determinate and indeterminate tomato plants while Sandy’s Garden has a good explanation of what a semi-determinate tomato plant is You can use the tomato's growth habit to your advantage if you factor it into the positioning of the plants in your garden.

On Temporary Greenhouses




My temporary greenhouse in December

Creating a temporary greenhouse can be quite rewarding as long as you can keep it sanitary and disease-free. I have only experienced the joy of growing normally warm season produce in the greenhouse once. There was a direct correlation between the growing of that tomato crop and my garden contracting Septoria. So since then I decided to minimize my exposure to disease by putting all my tomato transplants in pots. Though I have been doing well in starting tomatoes I am still experimenting with how to get an early start to my cucumbers. I have almost given up on that though – I can only experiment with winter planting so many times before I need to buy more seed.

Regular cucumbers have a tough time taking any cold. It seems that the slightest frost will wilt a cucumber seedling while tomato plants are a little more forgiving to cool weather.

A good location is a very important factor in determining where to place a greenhouse. A place that faces south and receives a lot of direct sunlight is very important. It also helps if you can position your greenhouse to receive radiant heat from nearby structures. That way your greenhouse area can be more resilient to drops in temperature. My greenhouse consists of a sheet of 5 mil plastic that has been placed diagonally against a south-facing block wall that receives full sun exposure. By placing plastic along the upper part on top of the block wall (with blocks securing the plastic on top) and placing the lower part of the plastic along the garden’s edge (with blocks securing the plastic on the bottom) I am able to secure most of the greenhouse. I can continue securing the plastic on the west perimeter of the garden with blocks though I secure the eastern part of the plastic with a piece of rebarb that is rolled up in the plastic to secure it from the wind because the eastern portion of the garden continues beyond my greenhouse. I made a skeleton of wood pieces that lie diagonally from below the lip of the block wall to the edge of my winter garden. These wood pieces help keep the plastic up and keep it from moving around so much in the advent of a mild winter windstorm. Besides the radiant heat from the south-facing block wall, I place clear plastic bottles inside the greenhouse to soak up the sunlight and keep the temperature within the greenhouse as moderate as possible. Water is very good at moderating temperature, though I find that a clear plastic bottle absorbs heat much better than the opaque plastic bottle does.
 

The inside of my greenhouse with waterbottles and tomato transplants in sunken pots.

My 2011 Winter Garden

The Kids' Winter Garden in the Foreground

For those of you who have not had a winter garden in the past let me tell you – it can be wonderful. Some people in Tucson only plant winter gardens because they do not want the hassle and maintenance that summer gardens often demand. Though growing may happen at a slower pace, the watering required is so much less than required during the summer in any arid landscape. Because most insects do not do well in freezing temperatures, if you remove leaf debris you should be able to keep most bugs absent of your crop.  The major setback associated with winter gardening is waiting for plants to get big enough to harvest. This fall I planted mustard greens (for my soil fumigation experiment), Imperial Star artichoke, Little Marvel peas, radish, Purple Dragon carrots (for seed), Chartwell Romaine lettuce, Viroflay spinach.

I try to only plant Patented varieties of vegetables, such as the Imperial Star Artichoke, after the patent has run out. That way, I have full rights every part of the plant I produce. I currently also have some tomato starts in with my winter crop in my winter garden greenhouse.



The rest of my winter garden that is not in the greenhouse.