Rakweed

Rakweed was an infectious plant from the planet Raxacoricofallapatorius.

Biology
Rakweed was a small green plant with a red flower at the top. As the name suggested, it was an extremely adaptable and fast-growing weed. It could grow nearly anywhere and at incredible speeds. Its DNA would adapt quickly to deal with threats and individual plants could also communicate with each other by a high-pitched screeching sound. This connection could be broken by another high-frequency sound at 1421.09 Hertz, which would result in the destruction of all plants and spores within the sound's range. The Rakweed could produce large amounts of spores. The spores served two purposes; to increase the Rakweed's number, and to infect life forms which were considered dangerous to the plant. Those who were affected by the spores would suffer a distinctive red and black rash, become weaker and enter into a deep coma which they might never waken.

Despite this, Rakweed was still edible to both Raxacoricofallapatorians and humans, and was known to be addictive to Raxacoricofallapatorians. (SJA: The Gift)

History
Tree Slitheen-Blathereen had genetically engineered a strain of Rakweed that grew faster than normal. He and Leef gave a potted Rakweed to Sarah Jane Smith as a "gift" for stopping the Slitheen, which she reluctantly accepted. Unknown to Sarah Jane, the Rakweed flowered the next morning, and as a result it spread, just as the Slitheen-Blathereen had planned. The spores' first victim was Sarah Jane Smith's son, Luke Smith.

The Rakweed spread quickly around the local area, threatening to engulf the whole of London in a matter of hours. However, Rani and Clyde discovered the plant's weakness, prompting Sarah Jane to have Mr. Smith transmit the sound of the bell across all audio systems in the affected area.

The Slitheen-Blathereen were addicted to Rakweed and would gorge themselves upon the food. Sarah Jane later used this to her advantage, having Mr. Smith replicate the bell's noise, destroying the Rakweed in the Slitheen-Blathereens' stomachs. This resulted in the creation of a deadly amount of methane gas, causing the Slitheen-Blathereen to explode or, as Clyde put it, "fart themselves to death". (SJA: The Gift)