Weeds are plants that do not need to be planted, watered, or fertilized to grow and they can take over your garden. Their seeds are red and black and will quickly plant themselves if left alone. Often times they will produce more than one seed as well as toxic fumes that make your Piñatas ill. Weeds have a negative value, as when attempting to sell placed weeds to Lottie she will actually charge you chocolate coins in order to remove them.
If too many weeds are left unattended then the weeds will take over your garden, degrading it by causing the value of the garden to plummet, all of the garden space by replicating themselves, and some kinds of weeds injuring your Piñatas if they get too close to them, attracting Dastardos to break your sick Piñata.
However, their tendency to produce multiple seeds make it useful in economic ventures because the heads of weeds sell for high prices, alongside the seeds that they drop. Selling the weeds themselves to Lottie or Costolot will cost chocolate coins as they have a negative base value, so be sure you sell only the seeds and heads. In Viva Piñata: Trouble in Paradise, the heads and seeds of weed plants have negative base values as well, and thus selling them will cost you chocolate coins.
The other use of these weeds are to satisfy a requirement of a certain Piñata species. You should exercise caution with these plants, as they can attract other piñatas to eat it which creates seeds that plant themselves automatically, creating a cycle.