Isolation Distance Requirements for Peppers

by Jeff McCormack, Ph.D.
Founder, Southern Exposure Seed Exchange

It is strange that the cultivated pepper is a major food crop yet there has been very little investigation of the relationship between natural cross-pollination (NCP) and isolation distance. In 1941 Odland and Porter wrote that, "Plant breeders and seedsmen disagree considerably in their opinions relative to the amount of natural cross-pollination in the cultivated pepper....In seed production, knowledge relative to natural crossing is a great aid in determining the isolation necessary in the seed plots" (4, p. 585). The lack of information is due partly to the number of variables affecting NCP such as location, time of year, changes in insect populations and climatic factors. Also, NCP is more difficult to study in peppers than in some other crops.

Isolation distance recommendations are based on the intended use of the seed. In other words, what degree of purity is sufficient for the intended purpose? For example, for certified seed production, very high purity is required (virtually 100%). Many large commercial growers of pepper seeds isolate sweet bell peppers by 1/4 mile, and hot varieties are isolated from sweet or hot varieties by one mile (1,7). For home gardeners wishing to save their own seed, the recommended isolation distances are much smaller. Several seed saving guides have recommended distances ranging from several feet (5) to 50 feet (2) to separation by the length of the garden or as far as practical (2,8). In these cases, the seed is being saved primarily for one's own use or for exchange among a few individuals, not for commercial purposes where there is potential for widespread distribution. In preservation efforts, such as that being undertaken by the Seed Savers Exchange (SSE), there is always the potential for widespread dissemination of the seed: therefore a greater attention to seed purity is essential.

The purpose of this article is to recommend an isolation distance suitable for preservation efforts, and to present some of the considerations involved in determining isolation distance.

I have grown out some pepper varieties from the SSE and elsewhere only to find that some of the varieties were impure. Impurity is defined as an unacceptable percentage of plants containing excess genetic variability. A high degree of genetic impurity is not necessarily undesirable. Again, it's a question of intended use. If the intent is preservation of varieties true to name, then genetic variability is to be discouraged.

Because so many gardeners are now involved in preservation efforts, and because 1/4 mile isolation distance may be difficult to achieve in practice, there must be some middle ground where the trade-off between purity and practicality is small. What is needed is an isolation distance recommendation which will give a reasonably high degree of purity (at least 98%, preferably 99% or better) for small plantings.

A misconception exists among a number of individuals who save their own seed. The misconception is that no crossing has occurred if the fruit and foliage of the first generation (F1 generation) or subsequent generations appear no different than the parental generation. This misconception can be illustrated by performing the following test. The test consists of growing a row of hot peppers next to a row of sweet peppers, both varieties having approximately the same shape and color of fruit and otherwise similar in appearance. The seed from the fruit of the sweet variety is then saved and planted. When the fruit of this planting (F1 generation) is eaten, a high percentage of these peppers will be found to be hot (due to the presence of a dominant gene received from the hot variety). One enthusiastic bite into a hot sweet pepper will illustrate that similarity in appearance does not mean absence of cross-pollination. Incidentally, the hot trait will not disappear in the next generation (F2 generation) or subsequent generations unless the hot plants are rogued out (each plant would have to be grown in isolation to do this). Instead the genes will "move around" in the plants of the subsequent generations. This experiment demonstrates the obvious results of cross-pollination, but the manifestations of NCP between two similar varieties will be less obvious, especially where recessive genes are involved. These recessive genes are not so easily removed from the population of plants resulting from the NCP of two different varieties, partially because these recessive genes are not readily identified.

What percent of NCP can be expected of two varieties of pepper grown side by side and how does isolation distance affect the percent of NCP? In a recent series of experiments (6) conducted over a two-year period in five commercial fields in New Mexico, test plants of one variety were transplanted into commercial plots of another variety. Tester plants were placed 12" (30 cm) from adjacent plants of the commercial variety. The average NCP was found to be 42% with individual plants having a NCP value as high as 91%. This percentage may be unusually high because the commercial cultivar outnumbered the tester plants, and it's not usual procedure to plant two different varieties in this fashion. In a similar series of experiments by Odland and Porter (4) it was demonstrated that NCP values ranged from 9 to 38% depending on the variety of pepper tested. In my own experience I have had peppers appear to cross at 6, 15, and 25 feet with barrier plantings in between. The pollinating agents in most of these examples are honeybees, bumblebees and halictid bees such as sweat bees. None of the studies available to me investigated NCP as a function of distance. The authors did not recommend isolation distances possibly because there are so many variables involved whose effect are only partially understood. Listed below are some of the variables involved and their effects on isolation distance.

Variables Affecting Natural Cross Pollination
and their Effect of Minimum Isolation Distance Requirements
Type of Variable Adjustment of Isolation Distance
Plants planted in blocks rather than long rows Decrease
Greater number of plants Increase
Greater number of varieties Increase
Variety differences (e.g. hot versus sweet) Increase
Large number of pollinators present Increase
Presence of one or more barrier crops Decrease
Presence of alternate pollen sources Decrease
Staggered blooming times between plants (seed collected from first blooms of first blooming variety) Decrease
Collection of seed from center of block plantings Decrease

If you are growing only two or three plants each of two varieties, you may be able to get by with a smaller distance, say 50 feet plus barrier plants. Hot peppers as a rule of thumb may require approximately four times as much isolation distance as do sweet bell peppers. Flowers of hot peppers have style and stigma (female reproductive structures) which protrude further beyond the anther cone (male reproductive structure). This situation is called stigma exsertion, and whenever the stigma is exserted, the flower is more likely to be cross-pollinated.

The Lend-Lease Act of World War II states that for peppers, 14 mile isolation distance is desirable between two varieties, but the distance should not be less than 150 feet plus a barrier crop between two varieties (1,3). Evidently the 150-foot distance plus barrier crops represents a compromise between purity and practicality. In the absence of hard evidence on NCP as a function of isolation distance (but using the information from the Lend-Lease Act), I am proposing the following guidelines for SSE members which should give 98% or better purity for small plantings. Here the intended use is for preservation efforts where fairly high purity is desirable, where the seed may not be grown out each year and where dissemination is limited. On this basis the following isolation distances are recommended for the SSE:

* Between sweet bell varieties 150' plus barrier crop*
* Between hot and sweet varieties (or between two hot varieties) 600' plus barrier crop

*Note: Additional distance is desirable between long-fruited sweet varieties and sweet bell varieties, or between two long-fruited sweet varieties.

If necessary, modifications of these distances can be made by consideration of factors presented in the preceding Table of Variables Affecting NCP. If you cannot achieve the recommended minimum isolation distances of 150 feet and 600 feet for sweet and hot peppers, there are other alternatives for keeping the varieties pure:

* Grow only one variety, and be sure to check the distance to peppers in neighboring gardens.
* Protect the flowers with tape before they open and then hand pollinate with a camel hair brush.
* Cage each variety under an insect-proof material such as cheesecloth or window screen. (Note: Be careful to screen all entrances since bees are quite adept at finding small openings.)
* Isolate varieties in time rather than space.

NOTE: This article was previously published by the Seed Savers Exchange and several international periodicals. It has been reprinted here without illustrations. To contact the author send mail to Jeff McCormack.
1. Borchers, Ed., Director, Virginia Truck and Ornamental Research Station, Virginia Beach, VA, 1984. Personal communication.

2. Bubel, N. 1978. The Seed Starter's Handbook. Rodale Press, Emmaus, PA

3. Hawthorn, L. R. and L. H. Pollard. 1954. Vegetable and Seed Production. Blakiston, NY

4. Odland, M. L. and A. M. Porter. 1941. A study of natural crossing in peppers (Capsicum frutescens). Amer. Soc. Hort. Sci. Proc. 38: 585-588.

5. Rogers, M. 1978. Growing and Saving Vegetable Seeds. Garden Way Publishing, Charlotte, VT

6. Tanksley, S. D. 1984. High rates of cross-pollination in chile pepper. Hort. Science 19(4): 580-582.

7. Villalon, Ben., Texas Agricultural Experiment Station, Weslaco, TX, 1984. Personal communication.

8. Whealy, K. 1983. Seed Saving guide. In: The 1983 Winter Yearbook. The Seed Savers Exchange, Princeton, MO