Hmmm, strange.
On mine, the steps to use it are (going from memory here)
1) connect to propane tank
2) make sure the flame knob is turned 'off'
3) open valve on propane tank
4) open knob on burner handle and light flame
5) adjust pilot flame via knob as desired
6) squeeze the 'inferno' handle to get full flame amount and release to go back to the pilot flame
And the flame size (and heat output) is waaay different between the walking cane type one running off of 1 lb propane bottles and the ones with the soup-can sized flame 'shroud' running off of 20 lb (or larger) propane tanks.
The 'big' ones put out over 400k Btu/hr (or something like that).
To the OP, kill the weeds with glyphosphate (or similar) and apply a weed preventer in the rocks (like Preen or other similar germination inhibitor stuff
http://www.preen.com/products/preen-garden-weed-preventer ). IMHO.