His body is composed of sentient vegetable matter.
The ability to nourish oneself, typically via an external energy source. In Swamp Thing's case, likely through a form of photosynthesis and composting.
The Swamp Thing's strength level is directly tied to his connection to the Earth. Occupying his most commonly used body, the Swamp Thing is capable of lifting volumes of mass many times greater than his own body weight. With additional power supplied to him by The Green, his ultimate strength level is nearly incalculable, dependent entirely on The Green's health.
The Swamp Thing has complete mastery over all forms of plant life and can command vegetative matter to do his bidding. Through this power, he is in constant communication with the very essence of the Earth, and serves as the Plant Elemental for the entire planet. Swamp thing was also formally the elemental of "all" the elements including Fire, Water, Earth, Air and Flesh.
The ability to commune with plant life and to control its growth and form. He can summon forth vines to entangle or snare, and so on. He has threatened on more than one occasion to cause the microscopic flora within his enemies bodies to grow out and kill them from the inside.
Swamp Thing can grow multiple copies of himself, but these are only animated vegetable constructs, which have no actions without direct orders from Swamp Thing.
Swamp Thing can enhance his mental capacities by growing a giant model of a organic brain, and then attaching that brain to his head. It take circa 2 hours in which Swamp Thing cannot move until the brain is disconnected.
Swamp Thing can grow orange tubers in his body; when eaten, it causes the victim to experience illusions (good and honest persons will have relaxing illusions; evil persons will have painful and terrifying illusions).
Theoretically so long as there is some vestige of plant life anywhere in the universe, the Swamp Thing will always have a vehicle through which to house his consciousness, rendering him effectively immortal.
The Swamp Thing houses his conscious mind inside of a humanoid body made up of compacted flora and vegetable material. At will, he can abandon this form and take up residence inside of another shell, grown from any plant life in any location. Even if the body he is currently occupying is destroyed, Alec's mind lives on in The Green, and will reconstitute a newly grown body. The Swamp Thing operated for several years before he even became aware of this ability. Eventually, it was the occultist John Constantine who revealed Swamp Thing's elemental nature to him, and instructed him in reconstituting his body.
Post-Flashpoint, after Alec decides to become Swamp Thing again, he gains the ability to grow fully functioning wings made of leaves.
He retains the memories of Alec Holland, who was a brilliant chemist and botanist.