Green Alkanet (Alkanet, Evergreen Bugloss)

• Common Name: Green Alkanet (Alkanet, Evergreen Bugloss)
• Latin Name: Pentaglottis sempervirens
• Family Name: Boraginaceae

This is another plant that acts as a nitrogen sponge.  It's also said to be a skin irritant because of the tiny hairs on the stem and leaves, so it's probably best avoided, but you need not worry too much if your tortoises accidentally nibbles some. 

Young Green Alkanet can look similar to Foxglove (which is toxic)  and Comfrey, so be sure you have identified your plant correctly.