This description is a series of metaphors that describe how the jungle looks - using existing earth objects to give an idea of what it looked like.
Please use this interpretation as a basis of understanding. What you imagine this planet to be like should be based on your imagination - there is obviously no single correct description to understand here.
It was a nest of octopuses,
Clearly, there are not actually octopuses here. The large growths of the forest are being described to look like this however. It gives the idea that the trees will not look like those on earth, instead - looking more like inteweved octopus tentacles.
clustering up great arms of fleshlike weed,
The impression this gives, is that each of the arms are bunched together (clustered), covered (or made of) weeds/plant matter that looks like flesh. Here we can get the idea that it's a very dense forest, and rather than earth-like leaves, the plants have more of a skin-like appearance.
Again the use of great arms suggests large growths that continue up, rather than the regular branched trees we see on earth.
Wavering means to move in a flickering, quick motion. It describes to us that these large arms of growth are not fixed stationary like stone - instead, there is movement. An alternate word could be "trembling".
flowering in this brief spring.
The key point of this story is focused around each part of Venus receiving only one or two hours of sunlight every 7 years.
On earth, when the sun first comes back from winter - we get a spring season, where the plants begin to blossom and sprout into life. This description emphasises that the plants are suddenly coming alive for this short period of time (their spring).
Whether the flowering should be taken literally, or as a description of the way that they suddenly appear to have come alive from their docile "winter" state - is completely up to your interpretation. The key idea is that they are suddenly more active and alive looking than before the sun reached them.