Krynoids were large, carnivorous alien plants that could be thought of as galactic weeds, (TV: The Seeds of Doom) a race of parasitic invaders that lived only to consume animal life. (PROSE: An Apple a Day...)
On planets where the Krynoid took root, all animal life would become extinct, as Krynoids desired that all plants should win against the animals that ate them. (TV: The Seeds of Doom)
Biology
Krynoids travelled through space as small seed pods, always in pairs (TV: The Seeds of Doom) which the Eleventh Doctor described as siblings and which had fleshy interiors. Although they didn't do well in the cold, snow and ice rendering them inert, (PROSE: An Apple a Day...) they were very hardy and could easily withstand the extreme cold of space. (TV: The Seeds of Doom)
A Krynoid itself was bulky, covered in tendrils and vines, some of which ended in red flower-like structures, (TV: The Seeds of Doom) along with tentacles and proboscises. (PROSE: An Apple a Day...) They had an undefined anatomy with no front or back. They required animal proteins while young, but could survive on normal photosynthesis later in life, and gathered nitrogen directly from the air without a need for roots. (TV: The Seeds of Doom) At a certain point in their germination, Krynoids manifested the ability (PROSE: An Apple a Day...) to channel their power into other plant life within a mile radius. They could also psychically possess other humans, though this may have been due to some humans' sympathy for plants.
They had great strength, and though they had great tolerance for cold, high temperatures could damage them. Additionally they were unaffected by gunfire and lasers, but could be affected by cuts as well as fighter plane missiles, (TV: The Seeds of Doom) and could be killed with special rockets. (COMIC: Don't Step on the Grass) One strain of Krynoids were affected by high-pitched noises, allowing the original consciousness of their host to reassert itself temporarily. (AUDIO: Hothouse)
Life cycle
Krynoids started life as small seed pods about twenty centimetres in diameter. Possibly due to the volatile nature of the Krynoids' home planet, many of these pods were launched into space, always in pairs, and these pods could travel though space for thousands of years (TV: The Seeds of Doom) before finding a suitable planet, (PROSE: An Apple a Day...) as Krynoids couldn't live in environments with high carbon levels. (PROSE: The Taking of Chelsea 426)
Once a pod reached a planet and was exposed to sunlight and more tolerable conditions, it would grow. When a creature came too close, the pod would release a long tendril which infected the victim, slowly turning it into a Krynoid, (TV: The Seeds of Doom) although if a Krynoid did not find a host, it could also take root in the ground. (AUDIO: The Root of All Evil)
The transformation began almost immediately, the Krynoid taking over its host's mind as skin and hair turned plant-like in colour and texture. Initially, the shock of this transformation kept the animal-Krynoid hybrid sedate during this early period, the hyrbid desperate for warmth and protein, which it absorbed as it converted the host, growing larger as it ate. Body temperature and pulse rate would drop severely and plant bacteria — schizophytes — appeared in the bloodstream, before the body was covered in green vines. Finally, all but the basic shape of the lifeform was lost as it completely changed into a mass of green vegetation covered in growths, the host's mind giving way to Krynoid sentience. When the Krynoid took over completely, it became mobile again. (TV: The Seeds of Doom) The Fourth Doctor theorised that amputating the infected area might slow the transformation rate, (TV: The Seeds of Doom) but the Eighth Doctor later confirmed that this would not have achieved anything, and that short of amputating the entire infected limb (provided that the infection site was a limb) within not more than around twenty to thirty seconds of initial infection, there was no way of treating a victim short of euthanising them before they could fully transform. (AUDIO: Hothouse)
The Krynoid could grow incredibly quickly, outgrowing a house in a few hours, and within a day reaching its full size which was as large as St Paul's Cathedral. (TV: The Seeds of Doom) After consuming local fauna, it would reach primary germination, (PROSE: An Apple a Day...) releasing hundreds of pods and reproducing a thousand-fold until Krynoids covered the entire planet (TV: The Seeds of Doom) in a matter of days. (PROSE: An Apple a Day...) They could also grow to thousands of metres tall, poking through a planet's upper atmosphere and discharging seed pods into outer space. (AUDIO: The Root of All Evil)
Intelligence
The Eleventh Doctor described Krynoids as a "race" of parasitic invaders, one Kyrnoid referring to "colonising" Trenzalore, with the Doctor speculating that "someone sent them" there. (PROSE: An Apple a Day...) The Krynoid ideology was directed towards the revolution of all plant life against the animals that ate them and depended on them to live, believing "the plants must win."
A Krynoid seen on Earth in the 20th century could speak English though assimilating its host, and attempted to bargain with humans for their lives. It spoke of its host as being part of the Krynoid, referring to "us" and "we". (TV: The Seeds of Doom) Hosts being consumed by a Krynoid could feel another presence with them, even if they were alone, (PROSE: An Apple a Day...) and would hallucinate about being in a jungle or rainforest, beginning to feel a part of it. (AUDIO: Hothouse) Krynoids possessing humans could not describe concepts unless their host knew the right words, and a Krynoid that infected a wolf was incapable of speech. (AUDIO: The Green Man)
History
The Krynoids were well known throughout the galaxy, though difficult to study because of their violent nature.
Incursions on Earth
Krynoids apparently had been seen in Britain prior to the 12th century, as carvings of a "green man" made of plants existed in a Catholic Church.
During the reign of Richard I of England, two Krynoid pods were activated in England and laid siege to a castle. One, taking a local medicine man as a host, was defeated by the Knights Templar wielding axes and its remains burnt. The other, taking a wolf as its host, was defeated using boiling oil and gunpowder created by Moses of Tyre. (AUDIO: The Green Man)
Two Krynoid pods arrived on Earth in Antarctica approximately twenty thousand years before the 20th century. They lay dormant until the 1980s, when one of them was discovered by a scientific expedition.
This pod hatched after being thawed out by an ultraviolet lamp. It infected Charles Winlett, one of the human scientists who had found it. The Fourth Doctor, having been alerted to the situation by the World Ecology Bureau, went to the base and investigated. He found the second pod. The second pod was stolen by Scorby, who destroyed the base and the Krynoid with a bomb and took the pod to Chase Mansion, botanist Harrison Chase's home.
Chase allowed the pod to infect one of his scientists, Arnold Keeler. This second Krynoid grew to gigantic size and developed a psychic link to Chase, who agreed with it that plants should be dominant over animals. The Krynoid was destroyed when the RAF bombed the mansion in an aerial attack. (TV: The Seeds of Doom) UNIT kept information on these Krynoids in the Black Archives (COMIC: Don't Step on the Grass) and pods were kept in the Vault. (AUDIO: Tales from the Vault)
The Eleventh Doctor once saved the actor Peter Davison from a Krynoid. (COMIC: The Girl Who Loved Doctor Who)
At some point around the end of the 20th century, two Krynoid pods landed in a field in Yorkshire. One took root in a farm field, overtaking and choking the crop; the other infected the farm's owner. The Krynoids were blown up with Diesel fuel by Eve Black and a local policeman. (AUDIO: The Root of All Evil)
In the 21st century, former musician Alex Marlowe led a project attempting to genetically engineer a new Krynoid strain that would cause the physiological changes induced by the Krynoid infection to still take place, including granting the power to control plants, while keeping the victim's original consciousness intact. The project was a failure. It produced a type of Krynoid with a far higher metabolism. It completed its life-cycle in a single hour, not the day normal Krynoids took. Fortunately, the new Krynoid strain had an unexpected weakness in that high-pitched sounds, rather than just causing it pain, allowed the victim's consciousness to re-assert itself for a brief period. The Eighth Doctor was able to exploit this weakness and destroyed Marlowe and his new Krynoids. (AUDIO: Hothouse)
A Virgoan space probe had an encounter with a Krynoid on Melandra IV, which resulted in it bringing the Krynoid to 21st century Earth. (PROSE: Stop the Pigeon)
The Twelfth Doctor recalled dealing with a Krynoid for Hyphen T Hyphen. (COMIC: Space Invaders!)
On Trenzalore
A pair of Krynoid pods landed on the planet Trenzalore. The Eleventh Doctor speculated that "someone" had sent them to Trenzalore. One of them lay inert in the snow, but another fell into a climate-controlled orchard inside a greenhouse, and subsequently hatched and infected the farmer Pieter. The Krynoid grew to the size of a barn and fed on live animals in its aim to colonise Trenzalore and consume all animal life. The Doctor and his companion Theol Willoughby drew the Krynoid to the Clock Tower in Christmas, where one of Christmas' townspeople, Jerl Tompkinson, used a rubber hose to fire water at the Krynoid, freezing it. With the Doctor's help, the townspeople pulled at a rope to ring the Clock Tower. The reverberations from this noise shattered the Krynoid's frozen body. The Krynoid's—and Pieter's—remains were burnt on a makeshift funeral pyre, along with the unhatched pod. Theol carved the Doctor a walking stick out of a piece of Krynoid for the Doctor to remember Pieter by. (PROSE: An Apple a Day...)
On Gallifrey
When Handrel was trying to give himself unlimited regenerations, he used a Time Scoop to capture a Krynoid for research. (PROSE: The Time Lord's Story)
Other universes
In a universe where the Doctor worked with a "new version of UNIT" after he was exiled to Earth in 1997, "an annoying man" tried to infect the United Nations with Krynoid pods. The Sixth Doctor and Evelyn Smythe accidentally travelled to this universe and helped the exiled Doctor save the UN. (AUDIO: The 100 Days of the Doctor)
Other references
After suffering from memory loss, Sarah Jane Smith could not tell the difference between Krynoids and Pescatons. (PROSE: Interference - Book Two)
The Tenth Doctor compared the swamp monsters to stage trees, comet flowers and Krynoids. (PROSE: Wetworld) He once thought an occurrence of hostile plants was caused by a Krynoid. He was proven wrong. (COMIC: Don't Step on the Grass) He later worked out that the Spores, which turned out to be Rutans, were not Krynoids. (PROSE: The Taking of Chelsea 426)
When the telepathic Emily Fairfax read the mind of the Hypothetical Gentleman, a Krynoid was among the images she saw. (COMIC: Hypothetical Gentleman)
Behind the scenes
The humanoid Krynoid costume was a modified Axon costume painted green. Ironically, plans for a collect-and-build Krynoid toy were abandoned and the Krynoid toy was instead painted orange and pink and turned into an Axon toy.