When we first seriously consider trying to obey all of Torah, as much as we can anyway, we’re confronted with commands which seem arbitrary and difficult, such as the law governing the kinds of materials used in clothes. It appears twice: “neither shall a garment mingled of linen and woollen come upon thee” (Le 19:19), and “Thou shalt not wear a garment of divers sorts, as of woollen and linen together.” (De 22:11)
The topic initially raises eyebrows for the submitted believer, since most of the clothing we wear today contains a mixture of fiber types. Garments of cotton and polyester are quite common, inexpensive, durable and comfortable. Why would God forbid clothing like this?
In order to get back to a reasonable place, we must, as always, read this command VERY carefully. In both instances, the kind of diversity forbidden is specified very explicitly and clearly; and for anyone who knows anything about textiles, it’s no surprise. The command does not forbid mixing just any two kinds of fibers, but fiber types that are distinctly different in fundamental, incompatible ways. The example given is linen and wool.
Textile manufacturers understand that wool and linen are fundamentally incompatible: linen is a plant-based fiber that creases easily and wool is an animal-based fiber that shrinks easily. The two types of materials also wear very differently and require very different types of care. Wool requires a different type of storage than linen due to susceptibility to mold and moths, and a different cleaning protocol to keep it from shrinking. When only a portion of the fibers of a garment shrink while other fibers don’t the entire structure and weave of the fabric is compromised.
Further, recent scientific studies have confirmed that wool and linen have very strong electrical properties when reacting with light, and these two materials happen to work in opposite polarities. By themselves, garments of either wool or linen tend to energize the human body electrically and even provide healing benefits, but together these materials work against each other and cancel each other out. This effect tends to sap the strength of the wearer and may even cause discomfort or blistering. Any garment made from these two materials would be difficult to care for and unhealthy to wear.
God knows what He’s doing when He gives us commands; His laws are good (Ro 7:12), but if we aren’t careful we may easily misapply them and cause ourselves and others needless inconvenience. (Lk 11:46) When we interpret Torah, we must do so lawfully (1Ti 1:8), being careful to keep it in full context, and in accord with the law of love. When God’s law appears overly burdensome, it’s likely we’re missing something basic: His yoke is easy and His burden is light. (Mk 11:30)
Here’s a link to an article about how orthodox Jews observe this command.
The fact that threads of gold fiber were woven into the priestly garments proves that God is not forbidding mixed garments of any two different types of fibers. “And he made the ephod of gold, blue, and purple, and scarlet, and fine twined linen. And they did beat the gold into thin plates, and cut it into wires, to work it in the blue, and in the purple, and in the scarlet, and in the fine linen, with cunning work.” (Ex 39:2-3)
The prohibition of plowing with an ox and an ass together is further evidence of the spirit of this command: “Thou shalt not plow with an ox and an ass together.” (De 22:10) The two animals are generally not matched in size, gait and strength, making the team awkward, inefficient, and perhaps harmful to at least one of the animals.
The same concept evidently applies to the sowing of seeds in a garden or field; the restriction is a common sense one. “Thou shalt not sow thy vineyard with divers seeds: lest the fruit of thy seed which thou hast sown, and the fruit of thy vineyard, be defiled.” (De 22:9) “Thou shalt not sow thy field with mingled seed.” (Le 19:19b) Mixing up different kinds of fruit-bearing seeds and planting them together would make it more difficult to care for and harvest the crop, especially if different techniques are required for different types of crops, while providing no other benefit. It is much more efficient to keep similar kinds of plants together in the same row or area of the garden. However, this would not forbid planting seeds for other kinds of plants in the same area which might ward off pests or provide some other benefit to the soil without hampering the crop or gardening process; these would be synergistic rather than incompatible.
In these commands, God is essentially communicating His love for us, telling us to be efficient and to take good care of ourselves in our physical lives, teaching us principles to help us in our social and spiritual relationships. God prescribes and encourages synergistic relationships, where different kinds of people engage to generate value and/or encourage each other, such as in business, marriage and in the church, but He tells us to avoid the unequal yoke (“Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?” 2Co_6:14), of which each of these commands is a physical illustration.