At first glance, this appears to be an ordinary walnut, though Detect Magic will reveal a hidden power.

If this walnut is thrown (improvised thrown weapon attack), a full-sized walnut tree will immediately sprout from the spot where it touches the ground. Anyone within a 10' radius of the spot must make a DEX save (DC 15) or take 3d6 bludgeoning damage and be knocked 15' away from the point of impact, and prone (save for half damage, pushed 5'). If the wielder botches the attack roll, the tree will sprout in their own square 

1d4 rounds after the tree sprouts, it will produce a crop of walnuts that immediately fall to the ground, making a 20' radius area Difficult Terrain.

Walnuts may be collected from this crop, and each walnut has a 5% chance of being another Wondrous Walnut.