The Mighty Mustard Green
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Fall greens are common in southern winter gardens. Cold-hardy greens in the Cabbage or Mustard family, also known as the Brassica family, are diverse in leaf shape, size, color, and flavor. Frequently cooked with ham hock or fatback, these greens feed many of us through the winter. This week I wanted to highlight one unique groups in this family- the mustard green.
The mustard green spectrum is broad- from the spicy Southern curled mustard, to broadleaves such as Tendergreen, purple mustards like Osaka purple, and many different Asian-type mustards, there is something for everyone in this group. Some are very mild while others are so “hot” that most people will shy away from eating them raw. Among the Asian mustard greens, some of the most popular are Bok Choy, Mizuna, and Tokyo Bekana. Milder mustards may enjoyed raw as a salad green or in stir-fry.
Mustards are some of the most cold-hardy greens, and within the mustards they vary in hardiness. Curly mustards are often more cold hardy than the broadleaf types, but they vary based on their breeding history. Pac choi, Chinese Thick-Stem, and New Star are some of the more cold hardy varieties. They will overwinter unprotected in USDA plant hardiness zone 7, which most of Wilkes County falls into. Keep in mind that while they survive winter, freezes can damage the leaves or make the plant die back to the crown. Protect sensitive mustards with row cover over winter to help them survive. Or, protect the more hardy varieties to maximize harvest through the winter.
Plants in the Brassica family contain compounds called glucosinolates. Glucosinolates are organic compounds derived from carbohydrates and amino acids that contain sulfur and nitrogen. Sulfur gives these plants their pungent odor and flavor. Glucosinolates break down into other compounds when they are exposed to an enzyme called myrosinase when the plant material is chewed, damaged, or cut. One of these compounds which is called isothiocyanate, or mustard oil. This compound is known for its anti-cancer properties, one reason why these vegetables are good for us.
When these compounds are volatilized with the addition of water, they form mustard gas. You may recognize mustard gas as a chemical warfare agent- indeed, it is highly toxic for us to breathe. And it’s not just toxic to us- it may also kill or reduce soil-borne pathogens such as fungi and root-knot nematodes.
Soil-borne fungi like Fusarium species and Southern Blight (Sclerotinia rolfsii) can and do live in our native soils. They may appear in patchy spots, in clay soils, and river bottoms. Sandy river-bottom soils are also more prone to having damaging populations of root-knot nematodes, microscopic roundworms that colonize plant roots and can inhibit plant growth.
Planting and managing mustard greens for mustard gas production may present an opportunity for biofumigation of soils with difficult-to-control soil-borne pathogens. To do this in a home or market garden, plant mustards in their appropriate season- early fall and spring. Allow them to grow as much as possible, to accumulate the maximum potential leaf mass. Harvest them throughout that time if desired. Keep in mind that the more leaf mass present when you mow, chop, and cover, the greater the content of mustard gas will enter your soil.
When you are ready to terminate them, mow with a flail mower, weed eater, or bush hog. Then cover the row or field of mustards with a tarp to trap in the mustard gas that forms as the mustards break down. Water helps with gas formation, so try to mow right before a rain or water the area before covering. If you cannot cover the area, you may still benefit from managing mustards as described and allowing them to breakdown prior to planting another crop. Allow at least one month for these crops to breakdown before you plant another crop. The biofumigation process may also inhibit weed seed germination for a short period of time.
For those with root-knot nematodes in the soil, summer-grown marigolds should be planted during the summer months as well, and managed to reduce nematode populations. While mustards are a good tool for limiting egg germination and initial propagation, but since nematodes multiply during the summer months, warm-season plants such as marigolds further reduce populations while nematodes are multiplying.
Not all marigolds will help control root-knot nematodes. Tagetes erecta ‘Crackerjack’ and ‘Flor de Muerto’ and Tagetes patula ‘Botanica mixed’, ‘Gypsy sunshine’, ‘Tangerine’, Scarlet Sophia’, and ‘Single Gold’ have been documented to affect root-knot nematode populations. If you aim to reduce nematode populations with marigolds, use 300 (350 seeds) plants per 100 square feet, 3,000 seeds (0.5 oz) per 1000 square feet, or 130,680 seeds (17.5 oz) per acre. Space marigolds approximately 7” apart and till in at the end of the season. Before you till, collect any seeds that may have formed for planting in subsequent years. Repeat in affected fields in alternating years. One opportunity for nematode reduction would be to grow both mustards and marigolds in one field for one year, rotating with vegetable crops. Likewise, certain mustards have a higher concentration of the compounds that can reduce soil-borne pathogens.
Mustards can often be grown for harvest after or ahead of summer crops, such as beans or squash. Sow fall mustards 8-10 weeks before the first frost to get strong top growth before the first frost. Sow spring plantings as early as the ground can be worked, in late January through early March, depending on the winter. Mustards often bolt, or initiate flowering, in warm weather. Direct sow the seeds ¼-1/2” deep, and keep moist through germination and establishment. Thin plants to 6” spacing after the plants have developed true leaves, for continued harvest. Or wait until they are a little larger, and thin via destructive harvesting, whereby the harvested plants are also eaten.
Take a soil test well before planting to obtain recommendations for fertilizer and lime. In absence of a soil test, apply one cup of a general fertilizer such as 10-10-10 per 10 feet of row ahead of planting. If you grow and harvest for a long period of time, you may need to supplement nitrogen fertilizer as well.
There are no silver bullets in gardening. But, growing mustards can reduce soil pathogens in home gardens where commercial fumigants are not available. In spite of their strong flavor- and actually because of it- mustards are truly a medicinal and beneficial plant. If planted as a soil cover for winter, they will also prevent erosion, capture unused nutrients in the soil, and reduce fertilizer need in the long term.