Chuck: do you have the Sep/Oct issue of Sport Rocketry? If you do, take a look at the NSL launch and notice the field, and that sparkies were used. Green grass that is short with little to no thatch will be safe. Longer grass/weeds etc. normally found in ditches could be green ( new growth) but have inches of dry dead grass (thatch) underneath the new growth. Besides a plowed field a sod farm should be one of the safest places to fly as far as fires go. They are not going to sell dead dried out grass to anybody that wants sod!
Back to the ditch. If it is in your launch area that needs to be clear( any grass over a few inches high,with lots of thatch, a normal ditch) and you have no choice by moving the pads further away, clear it. A gas powered "weed eater" with metal blade if needed, does wonders. Take it as low to the ground as you can. Rake up the cuttings get them out of the area. Once a year should be good depending on weather conditions. The new growth, again depending on weather conditions, will not burn as long as it is green. Keep your eyes on that area.
Yellowish grass in green grass will not burn. Brown,"crispy" grass will burn. How fast a fire will spread depends upon a few things. How dense the material is, the moisture content remaining in that material, and the biggest factor, the wind speed.
It's fall, lets use leaves as an example. A freshly fallen leave will not burn as fast as one that has been laying around a week losing its internal moisture. Ok, it's been a week we have nice dry crispy leaves on the ground. I have lots of green grass still showing between the leaves. I start a leave on fire, it burned the one next to it, both went out, had some green grass between them and the next leave. Now I'm going to rake some together about ten leaves deep. I have an area 3x3 feet of leaves ten deep surrounded by green grass. I started them on fire and they totally burned up until reaching the grass and went out. This was a windless day, I would hope everyone understands what the effects of the wind speed can do. Little or no wind with anything burning will mean a much slower spread.
I would clear out any high grass if located in an area that needs to be clear whether it looks like it needs to be or not. Take it down to the ground, rake it up, and go farther than required. And regardless of where you are launching never let that morning dew fool you into thinking it is safe to launch. Material with no internal moisture left will burn regardless of that moisture on top of it. Fly safe!!
I see Jim already has answerd your question, but will post mine anyway, have been working on it the last hour between chores here!
