Made this a few years ago but sadly couldn't finish it due to moving. I can't restart the idea at moment due to not having the land I had before so though I would share it with people on permies for people to try out.

Using the same principles as hemp crete I was using nettles as they were freely available, perenial and easy and legal to grow. Nettles are also very closely related to hemp and have lots of silica (the stinging part) which helps explain the strength. Initially I made a small test piece in a terracotta pot once it had dried and the terracotta removed it was possble for it to suport my weight.

Harvest the nettles
Dry the nettles and bash off the leaves.
Run the nettle stems through a mulching machine chopping them up
Mix the nettles with hydrated lime and press into blocks. I used a washing up bowl for this.
Let the hydrated lime dry and then remove the blocks

The plan after this was to create an entire wall and lime plaster this. I started this as I went along but then had to move. Obviosuly like hempcrete you will need to protect the blocks from being satured (with a roof and waterproof foundation) or they will fall apart. However they did last while I was at the property.

More pictures that didnt upload last time...