Another way is to grow a green-manure crop in summer. Sesbania, commonly used as a summer crop, thrives in hot weather. The frequent and heavy irrigation required for a summer crop kills most weed seeds in the surface soil. The green-manure crop is plowed shallow or, better, disked into the surface when the soil is still moist. With heat and moisture, the green crop rots rapidly.

A third method, growing the flax in cultivated rows, has been used successfully but not widely in the Imperial Valley. For row planting, the ground should be well prepared by irrigation and tillage, as for ordinary seeding. The seed is sown in drill rows 18, 21, or 24 inches apart, the spacing determined by the tread of the tractor and the type of cultivator used. The seed should be sown 24 to 30 pounds an acre so as to obtain a thick stand that will check weeds in the rows.

Shallow cultivation should be given as soon as possible after the first and second irrigations. Two cultivations are usually sufficient to eliminate most weeds until the flax is bolled. Timely harvest will then prevent any late weeds from going to seed. Where flax is grown for sale as certified seed, the few weeds that escape in the rows can be pulled by hand.

The broadleafed annual weeds can be checked or killed by spraying with Sinox or other selective spray material that adheres to the rough hairy surface of many weeds but does not wet the smoother leaves and stems of flax or grasses. Sinox is of no value in the control of wild oats, cheat, darnell, or other grass weeds. For best results, spraying should be done when the weeds are very small and the flax is 4 to 6 inches high. In no case should the spray be applied after the flax plants reach the flower-bud stage; at that stage, serious injury may occur. Sinox is most effective on clear warm days when the temperature is above 65° F.

Fifteen years of experiments at the Imperial Valley station indicate that early November is the best time for seeding. The highest yields and quality have been obtained from seedings made from November 1 to 20. In general, there is a reduction in yield when seeding is delayed until mid-December or early January, apparently because of the shorter growing season of the later planting.

Flax sown at different dates, October to December, ripens at about the same time; that is, in late April or early May, when high temperatures force maturity of the crop. Flax sown early in November has a long blossoming period (30 to 50 days), and a growing season of 150 to 180 days. These conditions permit the maximum setting of seed bolls and full maturity of the crop. In the date-of-seeding tests at El Centro, November plantings have shown, on the average, taller plants, a longer blossoming season, larger seeds, a higher oil content, and a higher iodine number of the oil than obtained from December plantings.

In the Imperial Valley it is a common practice to seed flax following alfalfa, sugar beets, carrots, lettuce, or another vegetable crop. Perhaps the best soil preparation for flax is a green-manure crop of sesbania, guar, cowpeas, or clover. These improve the soil, help maintain the nitrogen, control weeds, and reduce the hazard of diseases that may be carried over in the soil. Two or more crops of flax are often grown in succession. Generally, however, this is not a good practice, because both weeds and flax diseases are apt to increase under continuous cropping.

The use of fertilizers depends to some extent on the previous crop or the rotation system followed. Both phosphorus and nitrogen are usually needed for maximum yields. A safe rule is to apply about 65 units of available phosphate before planting, and 32 units of nitrogen in the first irrigation after the flax is up, or any time before the flower buds appear.

The success of flax in California and Arizona has been due to several factors: The excellent adaptation of the Punjab variety as a winter crop under irrigation in the area; the skill, resourcefulness, and cooperation of the pioneer growers of the crop; and a ready market for the high-quality seed. Perhaps the world record for yield of flaxseed was made by Harrison Emrick, of Yuma County, Ariz., in 1939. His field of 24.7 acres of Punjab flax made the remarkable yield of 61.6 bushels an acre.

THE AUTHORS

Arthur C. Dillman, formerly an associate agronomist, Division of Cereal Crops and Diseases, in the Bureau of Plant Industry, Soils, and Agricultural Engineering, is now an agronomist with the Flax Development Committee, of the Flax Institute of the United States, in Minneapolis.

L. G. Goar is an associate in the Experiment Station (Agronomy), California College of Agriculture, and is Superintendent of the Imperial Valley Experiment Station, El Centro, Calif. He has worked with flax and other oil-bearing crops continuously since 1926.