Jasbird wrote:

After spraying weeds and grass with paraquat - how long is it
before one notices whether it has worked? And what happens to
the plants? Do they turn yellow first? My plants don't look any
different to how they did 2 days ago when they were sprayed.

They are probably ice cold and barely metabolising. Paraquat and most
other weedkillers only take effect when plants are in active growth.

If I tried spraying my plants now it would freeze on contact - the soil
is like concrete.

You might see something happen eventually when it warms up a bit and
growth restarts but the active ingredients will lose some potency in the
meantime. Using weedkiller during cold snaps or immediately before heavy
rain is wasteful! The instructions should advise minimum temperature for
effective use.

Regards,
Martin Brown