"I look back across the years to the seven-year-old wizard who spent hours in his bedroom dismembering Horklumps and I envy him the journeys to come..."
-- Newt Scamander from the introduction to Fantastic Beasts and Where to Find Them
Horklumps are garden pests, originating in Scandanavia but now found throughout northern Europe, that resemble pinkish mushrooms covered with bristles. Horklumps are actually a carnivorous animals rather than plants. Horklumps eat earthworms and are the favourite food of gnomes (FB). They have sinewy tentacles and will quickly overrun an average garden. Streeler venom, which is highly toxic, can be used to control an infestation of Horklumps (BoP).
The Horklump is listed as a pest by the Pest Advisory Board (JKR-W1).
There is some conflicting information as to the usefulness of the horklump. Newt Scamander says that the creatures have no use, whereas horklump juice can be collected in Pottermore, presumably to use in potions. It is likely that the use of horklump juice was discovered after Scamander's time, and thus is not mentioned in his book.
From the Web
"Horklump" on the Harry Potter Wiki