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Has anyone tried Sunn Hemp (Crotalaria juncea) as a green manure?  RSS feed

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Location: Central Oklahoma (zone 7a)
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TO:  Karl Trepka
FROM:  Eric Koperek = erickoperek@gmail.com
SUBJECT:  Sunn Hemp (Crotalaria juncea)
DATE:  PM 4:02 Thursday 15 September 2016

(1)  Seed for Sunn Hemp (Crotalaria juncea) is costly in the United States.  There are other cover crops like Red Clover (Trifolium pratense) that fix lots of nitrogen but are much less expensive and better adapted to northern temperate climates.

(2)  Sunn Hemp will not set seed in northern latitudes = the plant is daylight sensitive.  The only places in the United States where Sunn Hemp will produce seed are Hawaii and southern Florida.  Translation:  You cannot grow and save your own seed so Sunn Hemp is NOT a "sustainable" choice for farmers in most temperate climates.

(3)  Sunn Hemp is a tropical crop best suited for regions with long, warm growing seasons.  Minimum soil temperature is 65 degrees Fahrenheit.  70 degrees is much better.  Sunn Hemp is NOT recommended for cool or short season climates.

(4)  Sunn Hemp is great for Mulch-In-Place agronomy in places like Brazil and other hot, wet climates (or hot, dry climates if you have plenty of irrigation water).  We have used Sunn Hemp on our tropical plantations since the mid 18th century = 1750's or so.  The cover crop overwhelms weeds and fixes about 200 pounds of nitrogen per acre each year.  Seed densely, then cut when mature with a sickle bar mower or knock down with a roller-crimper.  (When I was a boy we used large field crews with razor sharp machetes).  Hand plant or use no-till equipment to drill seeds or set transplants directly through the mulch

(5)  Note that Sunn Hemp is a broad leaf plant that rots relatively quickly.  If you have a really serious problem with aggressive, perennial, deep-rooted weeds it is better to use a grass-species cover crop like Sudan Grass (Sorghum sudanense) or Forage Maize (not silage maize) = Zea mays or Cereal Rye = Winter Rye (Secale cereale).  Long grass straw rots much more slowly and so provides better weed control.  You need a tight, nearly impenetrable mat about 1/2 inch thick = 8,000 to 10,000 pounds = 4 to 5 tons of long straw per acre to obtain 90% weed control for 3 to 8 weeks (just enough time to get your crop up and well established.  Once crop canopy closes weeds are no longer a problem). 

(6)  I use Sunn Hemp on my vegetable farms near Homestead (south of Miami, Florida).  Sunn Hemp is a good rotation crop for killing nematodes which are a BIG problem in tropical and subtropical climates.

(7)  In northern temperate climates Red Clover (Trifolium pratense) is a much better choice than Sunn Hemp because clover seed is dirt cheap.  Red clover will blot out any northern weeds and fix 150 pounds of nitrogen per acre (if you let the crop grow for a full year).  You can seed corn directly into standing red clover.  Mow clover first with a flail or rotary mower then use a no-till seeder for planting.  Mow clover again 2 weeks later just as the first corn seedlings start to emerge from the soil.  You can grow 150 bushels of dent corn = 8,400 pounds = 4.2 tons per acre in red clover without any herbicide or fertilizer in areas with 40 to 45 inches of rainfall yearly.  Higher yields are possible if you irrigate.      

ERIC KOPEREK = erickoperek@gmail.com

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