I would hesitate to call “tendergreens” simply mustard or mustard spinach, but perhaps to use the scientific name of Brassica, but the tendergreens may cover various species within the “Brassica” family.

https://www.google.com.jm/search?q=spinach mustard tendergre...

http://www.memidex.com/tendergreens
Brassica
a genus of plants in the mustard family. The members of the genus are collectively known as cruciferous vegetables, cabbages, or mustards. Crops from this genus are sometimes called cole crops, which is derived from the Latin caulis, meaning stem or ... (41 of 1326 words, pronunciation, 1 image)
en.wikipedia.org/wiki/Brassica [cite]

http://www.thefreedictionary.com/tendergreen
Noun 1. tendergreen - Asiatic plant cultivated for its swollen root crown and edible foliage
Brassica perviridis, Brassica rapa perviridis, spinach mustard
crucifer, cruciferous plant - any of various plants of the family Cruciferae
Brassica, genus Brassica - mustards: cabbages; cauliflowers; turnips; etc.



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Note added at 1 hr (2016-11-23 00:34:18 GMT)
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tendergreen, tendergreens- WordWeb dictionary definition
www.wordwebonline.com/en/TENDERGREEN
spinach mustard, Brassica perviridis, Brassica rapa perviridis. Derived forms: tendergreens. Type of: crucifer, cruciferous plant. Part of: Brassica, genus Brassica.
tendergreens | From the Land
https://pccsa.wordpress.com/category/tendergreens/
Jan 25, 2012 - Posts about tendergreens written by Prescott College Community Supported ... tendergreens: Brassica rapa var. perviridis or komatsuna.