Rumex acetosella (Sheeps Sorrel, Common sheep sorrel)
Part used: any
Time harvested: any
Colour range: yellow, green, gold, brown, olive, others
Mordant: non needed
Light fastness; Excellent
WOF: 20-30%
For mordanting 15%
 
Varying colours;
copper mordant will give great greens.
Ferrous sulfate; browns
Varying Ph level may give other shades.
Active dye components are anthraquinones and oxalates.
 
Hot dyeing: yes
Cold dyeing: no
Low Impact method: no
 
Our sorrel is grown in Hungary, 100% organic. 
Sorrel contains a lot of oxalic acid and as such, does not need a mordant, but make sure you dye in a well ventilated space.  You can use it as an oxalic acid mordant underneath other dyes and it would be less strong in colour than for example rhubarb roots.
Dyeing is quick, the powder dissolves easily in the water, just add the wetted fabrics and start heating. In under 30 minutes in hot water you can see the beautiful hues on your fabric
Use with alum mordant for warm goldens, ferrous sulfate gives you delicious greens.