Most rural farms have their own water source which might pump 10 gallons per minute. A typical fire hose will be using 100 gpm. Unless a fire is very small, you are not going to do much with a garden house. A municipal system with high pressure water might have enough force in a sprinkler system to put out a fire.
Most barn fires smolder for quite a while before the erupt into fire.
Then most barns burn very fast. Promise not to hold you responsible. Just looking for ideas from someone who knows a bunch more about fires than the rest of us. This is why barns and churchs burn faster than houses. Any place that you can put up a wall will delay a fire.