Horse chestnut - Aesculus hippocastanum

Read more about the tree here.
Note that I have only experimented with the ordinary Aesculus hippocastanum, so I have no idea whether the other varieties may be used for dyes.

Leaves are gathered AFTER flowering, but before the chestnuts ripen. Pick only freshly green leaves - brown edges and spots mean that less dyestuff will be present.

You will need 1000 g leaves and their stems for 250 g yarn/cloth. Shred lightly, place in cooking pot and cover with water. Bring to a rolling boil, lower heat and keep boiling for two hours. Top up with water as necessary. Turn off heat, and allow to cool overnight.

Strain off plant matter and discard. Place mordanted yarn/fabric in pot and bring to a slow simmer, just below boiling. Keep at 90 degrees centigrade (roughly) for at least one hour. The longer you keep the heat on, the more permanent the dye will be - up to one hour and 45 minutes. Turn off heat, and allow yarn to cool off in dyebath, preferably overnight.

Transfer yarn from cold dyebath to clean rinse water and allow to 'rest' there for 30 minutes.

Dry in fresh air and shade.

When dry, wash gently in soapy water and rinse carefully in three changes of clean water. Finish with a rinse in clean water with 1% vinegar added. Again, dry in fresh air and shade.

This colour scale was made with horse chestnut leaves in a copper pot. Left to right:
1: White Spelsau wool, 18% alum mordant
2: Light gray Cheviot wool, 15% alum + 2% tin salt
3: White Spelsau wool, 15% alum + 2% ferrosulphate

The paler fawns were dyed in the remains of the bath, for later overdyeing with oak leaves.