Notes from the Nails: As a kid, I wanted to be a tree when I grew up. They seemed so reliable and enduring and wise, with hundreds of years rooted to one spot, free to think in peace. It kind of surprised me that there isn't already a druid circle for this, so here we go...
Circle of the Root and Stem
The druids of the circle of the root and stem revere trees and plants above all, identifying them as the foundation upon which all of nature's glory is built. These druids tend to vast and beautiful woodlands, taking painstaking, personal care of every single plant that grows within. The circle's most powerful druids strive to become trees themselves, using their divine magic to transform their bodies into rugged towers of bark and leaf, the better to guard their sacred forests.
Leaf Mask Transformation
Starting when you choose this circle at 2nd level, you can use your Wild Shape to transform into plants as well as beasts. You cannot use this ability to become a spore servant of any kind, however.
While transformed into a plant, you can cast the cantrips shillelagh and thorn whip, requiring no material components, even if you would not normally know them.
Also at 2nd level, your skin takes on a rough, bark-like appearance, studded with sharp thorns. Your AC can't be less than 16, regardless of what kind of armor you are wearing. Also, whenever you are hit with a melee weapon attack, you can use your reaction to deal 1d4 plus your Wisdom modifier piercing damage to the attacker. These effects apply while you are Wildshaped.
Starting at 6th level, you can tap into the ancient wisdom of the trees. Whenever you would make an Intelligence check to recall lore or past events, you can make a Wisdom (Nature) check instead.
In addition, you can cast the spell speak with plants at will, without using a spell slot. This spell does not count against the total number of druid spells you can prepare.
At 10th level, your connection to the magic of the earth is so strong that you gain a measure of its solidity. You can't be pushed, shoved, knocked prone or subjected to any other form of involuntary movement as long as your feet are on natural ground. You also have advantage on Strength checks and Strength saving throws.
By 14th level, you have finally learned to become a tree. You can now use your Wild Shape to transform into an awakened tree, or you can expend two uses of your Wildshape to transform into a treant. While in this form, you add your Wisdom modifier to damage rolls for your slam attack, which is considered magical for the purposes of overcoming resistance and immunity.
Changelog: 3/6/17: Leaf Mask Transformation: Cantrips can be cast while you're whildshaped
Oaken Flesh: Renamed Thorny Flesh -- 1d4 +
Deep Roots: No longer requires a bonus action
Arboreal Form: You can use two uses of Wild Shape to become a treant