Comfrey.

Comfrey (Symphytum species) is essential to organic gardening.:

Mulch
Use the leaves in layers about 5 cm thick over the surface of the soil.
The leaves break down rapidly and provide nutrients to the crops.
Comfrey is particularly good for fruiting crops because of the good levels of potash.
When planting potatoes, lay comfrey leaves in the bottom of the trench and place the potatoes on top.
Or plant clumps of comfrey in the orchard. The leaves are then regularly slashed or mown and left on the surface of the soil to break down.

Comfrey liquid manure
Fill a large container with comfrey leaves, cover with water and a lid and leave for four weeks.
Leaves readily de-compose making a very useful (if extremely malodorous) liquid manure.
This fast-acting liquid food can be used as it is on established plants or diluted about 5 to 1, and used for seedlings, pots, and ailing plants.

Alternatively, firmly pack comfrey leaves into a wide pipe, weight with a stone and suspend the pipe above a small container.
The pipe should be blocked at the bottom end with just a small hole in the middle.
The comfrey leaves rot down releasing a dark liquid that collects in the small container.
Dilute this by about 15 to 1 and use in the same way as liquid manure.

Compost activator
Comfrey added to the compost not only rots down quickly but also adds nutrients and speeds up the decomposition of other materials.
I grow one clump of comfrey near the compost to remind me to add a leaf every time I add other materials.
Other plants in the Boraginaceae family, like borage and forget-me-nots, also break down quickly in the compost.

Weed barrier
Comfrey is used as a weed barrier because it is able to stop running grasses from spreading.
It needs to be planted in a strip several plants wide and it’s important to use only non-seeding, non-spreading varieties otherwise you will only be replacing one weed problem with another.

Slug and snail trap
Comfrey growing in big tubs to keep it under control
Grow comfrey near the compost
Big comfrey leaves are attractive to slugs and, to a lesser extent, snails.
So try layering the leave around but some distance from young seedlings.
Each morning pick up the leaves and remove the slugs and snails that are sheltering and feeding there.
Drop these into soapy water and replace the leaves to trap more slugs.