From Barlowe's Comprehensive Field Guide to Ephemeral Life Forms:
Grudges most frequently have the appearance of grassy plants, similar to sedges or sawgrass ... There are at least three known cultivars of grudges. The Common Grudge is rather delicate, and requires significant attention and frequent nursing. If appropriately cared for, it can transform into a Longstanding Grudge which can survive exposure to the elements. Harboring a Longstanding Grudge is not difficult; it requires only a minimum of attention. It is quite hardy as long as it can obtain a sufficient supply of enmity to prevent its withering away completely.
... A Grudge which receives a sufficient quantity of hatred and malice during its nursing will transform into an invasive form, known as Rancor. Rancor can spread widely and rapidly, often overwhelming entire neighborhoods and occasionally consuming whole towns. Eradication is challenging: not only can Rancor propagate from a single small cutting left behind, but it is capable of remaining dormant for years until conditions are right for it to emerge again.