What were the tripods spraying and what does it have to do with the red weed in War of the Worlds?

The red mist was blood. That's why Tom Cruise's character was so freaked out when he saw it running down the walls in the basement. The tripods were essentially grinding up the humans and spraying the pulp on the ground.

In the book, along with the fighting tripods, there were harvester machines that gathered the humans and placed them in metal baskets. The captured humans were processed and injected directly into the alien's veins for food.

This film version kind of mixed the two machines into one, whereas the original never even touched on it.

The red-weed you're questioning about resembled a veinous system in appearance. I did read somewhere that it was actually supposed to be Martian vegetation, although the film never explicitly says that.

The red mist is blood. They would take the blood and practically spit it out and use it for nutrients. The red plant you see every where is called the red weed. This is Wells' explanation for why Mars had a red appearance. After 15 days, the narrator said that the plant started to get gray spots and eventually gray all together and extremely brittle.

A lot of people didn't really get this part and it was a subtle treat for fans of the broadcast and book.

May be they are watering their plant/weeds with human blood or use it as fertilizer. The aliens may become short of supply in their planet and found our planet suitable for them. So, they sent the tripods to invade us and to make their food, they use us as water/fertilizers.

H.G. Wells wrote this novel in terms of the late 19th century. He was always looking ahead (an early science-fiction genre). So a lot of what he wrote would've been imagination. OK: What we understand of Mars today, bears no relation to Wells' generation's perception of the Red Planet. It was the Outer Limits Of Exploration, and anything could come from there. Giving rise to all manner of horror stories - War Of The Worlds included. The Red Weed was reputed to have given the planet its red appearance, which we now know to be nonsense. (Which still doesn't disqualify War Of The Worlds from being a literary masterpiece.) The only spraying done by the Martians in the original novel, was a "black mist" nerve-gas. Whatever the movie-makers invented, could have any number of explanations - including, that blood-drained corpses were then liquefied and spread as Red Weed nutrients.

To directly quote the book regarding the "red weed":

The last salient point in which the systems of these creatures differed from ours was in what one might have thought a very trivial particular. Micro-organisms, which cause so much disease and pain on earth, have either never appeared upon Mars or Martian sanitary science eliminated them ages ago. A hundred diseases, all the fevers and contagions of human life, consumption, cancers, tumours and such morbidities, never enter the scheme of their life. And speaking of the differences between the life on Mars and terrestrial life, I may allude here to the curious suggestions of the red weed.

Apparently the vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint. At any rate, the seeds which the Martians (intentionally or accidentally) brought with them gave rise in all cases to red-coloured growths. Only that known popularly as the red weed, however, gained any footing in competition with terrestrial forms. The red creeper was quite a transitory growth, and few people have seen it growing. For a time, however, the red weed grew with astonishing vigour and luxuriance. It spread up the sides of the pit by the third or fourth day of our imprisonment, and its cactus-like branches formed a carmine fringe to the edges of our triangular window. And afterwards I found it broadcast throughout the country, and especially wherever there was a stream of water.

The way I see the Red Weed is H.G. Wells hypothesised that Mars' red appearance came from predominantly red vegetation, rather than our Earthly green.

So when they came to Earth they were terraforming with the red weed to make Earth more Mars-like. Even though Wells was wrong about the reason for Mars being red, his idea for transforming a host planet was absolutely revolutionary.

I've read all the answers but I also think and I agree with feeding off human blood etc..? But I think the red veins or roots may have been like roots of a plant/trees sucking up all the nutrients of the earth?