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Dec 19 2012

Islolation Distances by Nick Guthrie

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From the February 2008 Newsletter

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One part of our mandate in CVGSS is to exchange vegetable seeds which breed “true”, that is, will go from plant to seed to plant giving a reliable production of the same features for generations to come. Of course, in any oneseed envelope there will be small and acceptable variations in size of fruit, resistance to disease, maturation date and so on. These small variations actually give the plants protection against climate variations and new diseases.Hybrid plants and the seeds which produce them are from man-made intentional crosses in which two plants are used as parents because the breeder wants to combine two or more specific traits into one offspring. Sounds reasonable, eh?See sidebar for a run down on genetics.Back to plants: the point here is that while hybrid plants may have a desired set of features in the first generation, the next generation will lose that advantage. Some of the plants will be ok, and the others won’t be. So-called “open pollinated” plants are those which can freely interbreed with others from the same seed package and reliably produce plants which are very similar to the parent parents one generation after another. These plants are said to breed true to type.However we must be careful to ensure that our favourite carrot plants from which we gather seeds must not be wind pollinated by the plants in the neighbours’ gardens, nor by Queen Anne’s Lace plants which grow abundantly in the ditches of the Valley.The attached chart lists isolation distances which must be observed if we intend to control the genes which go into our envelopes. Unwanted cross pollination can also be prevented by using Reemay or other cloth row cover which will block the entry of unwanted pollen.

Gregor Mendel and his Peas:

Mendel studied pea shape to determine how genetic traits were passed down. He had two pure breeding lines of plants: one that always produced round peas and another that always made wrinkled ones. He crosspollinated these two parental lines and found that every F1 hybrid plant had round peas. Therefore the round pea gene is a dominant gene. Mendel showed that the result was the same whether the pollen from plants with round peas was used to pollinate plants with wrinkled peas, or the other way around. The consistent results from such “reciprocal crosses” led Mendel to the conclusion that is now called Mendel’s First Law: factors that determine physical traits are present in both the male and female gametes.

Mendel’s next experiment was to self-pollinate each F1 hybrid plant with its own pollen this (F2) generation contained both round and wrinkled peas, in spite of the F1 plants having all round peas. In other words, traits that had been hidden in the F1 hybrids had reappeared in their offspring. Mendel found that the ratio of these two traits was roughly three round peas for each wrinkled pea.

Another way of explaining inherited traits is the Punnett square which illustrates how genetic traits are passed down to offspring.

Punnett Square Maternal
B b
Paternal B BB Bb
b Bb bb

Imagine that B is the round pea gene, which will cancel out the recessive wrinkled pea gene b in this F2 illustration.

Vegetable
Cycle
Pollination
Pollinator

Isolation

Distance

Seed

Longevity

Notes
Bean
A
Self
100′
2-3 yrs
Lose vigor rapidly.
Soybean
A
Self
100′
2-3 yrs
Space farther apart than for market crops.
Beet/Chard
B
Cross
Wind
1/2 mi
3-5 yrs
Beets cross with chards.
Broccoli
Kale
Cauliflower
B
Cross
Insects
1/2 mi
3-5 yrs
Many brassicas easily cross-pollinate. Learn more.
Carrot
B
Cross
Insects
1500′
2-3 yrs
Crosses with wild species…Don’t Save Seeds
Celery
B
Cross
Insects
1500′
2-3 yrs
Corn
A
Cross
Wind
1/2 mi
2-3 yrs
All local dairy & beef farms grow GMO corn – don’t trade seeds.
Cucumber
A
Cross
Insects
1500′
5-10 yrs
Many cucs, melons, squashes etc .inter breed. Special care!!
Eggplant
A
Self
150′
2-3 yrs
Leek
B
Cross
Insects
1500′
2 yrs
Onion
B
Cross
Insects
1500′
1 yr
Lettuce
A
Self
50′
2-3 yrs
Start indoors, need long season for seed.
Melon
A
Cross
Insects
1500′
5-10 yrs
Muskmelons will not cross with watermelons.
Mustard
A
Cross
Insects
1/2 mi
3-5 yrs
Crosses with wild species – they are also brassicas.
Pea
A
Self
50′
2-3 yrs
Pepper
A
both
Insects
500′
2-3 yrs
Radish
A
Cross
Insects
1500′
3-5 yrs
 Will cross with other radishes only
Spinach
A
Cross
Wind
1/2 mi
2-3 yrs
Squash
Pumpkin
A
Cross
Insects
1500′
2-5 yrs
*See cucs, above
Tomato
A
Self
25′-100′
5-10 yrs
Potato-leaf types need the greater isolation distance.
Cycle: A=annual, B=biennial.
Pollination: Self=self-pollinated, Cross=cross-pollinated by another plant.
Isolation Distance: recommended distance by which different varieties must be separated to prevent unwanted cross-pollination.
Seed Longevity: Averages, not guarantees. Seed longevity depends on the conditions.  Very dry and very cold are best.

Dan Jason of Salt Spring Seeds, says that Beans, Lettuces, Peas and Tomatoes can be assumed to self-pollinate even at close quarters.

Edited March 9, 2013 to match latest information from Nick Guthrie /dg

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