Armor

Uncommon

(requires attunement by a druid or ranger)
  This twisting mass of tightly knotted thorns forms a functional buckler. Its thorny limbs move ever so slightly to defend against incoming attacks that might otherwise pierce it. When you don the shield, the vines grow harmlessly around the arm and hand that's holding it, preventing it from being unwillingly dropped or disarmed. As an action, you can speak the shield's command word and touch it to the ground at a point at your feet. When you do, you doff the shield as the vines release from your arm and rapidly grow across the ground, creating a visible line of thorns and brambles 20 feet long and 15 feet wide. The thorns recreate the effects of the spike growth spell within that area, without requiring concentration, and last for the spell's duration. When it ends, the thorns coalesce back into the twisted shield. You can speak the shield's command word again as an action while you're within 5 feet of the thorny area to end the effect early, causing the vines to regrow onto your arm as you don the shield again. Once this property of the shield has been used, it can't be used again until the next dawn.