You should be able to take a .40 sized plane off with a .30 engine, so the LA should be enough.  However tall grass is another thing.   Does your club not mow the grass short?  They should be.  If the grass is short, then is the engine performing?  You should be fine with 11,000 RPM on a 10-6.  An 11-4 prop would be better from grass.

Also the plane and technique.

Make sure the wings are level with the ground front to back when resting on its gear, or just very slightly down.  Too much down force and it may have too much drag to take off, and when it does it will leap off and could stall.  If the leading edge is higher than the trailing edge it could take off prematurely and stall.  The stall could be at a very low altitude like fractions of an inch, may give a hopping effect. 

For the take off technique on grass or dirt fields.  Be sure not to use down elevator during the take off run, this will cause aerodynamic drag on the wing, and cause the wheels to dig in, both will slow the plane down.  Also do not use up elevator as this also caused aerodynamic drag.  Just keep the stick in neutral.   In fact keep it in neutral till it lifts off the ground, or appears to be at max speed ready to take off.  Now pull the stick back ever so slightly.  When off the ground ease up on the backpressure on the joystick.  Let the model level off and gain airspeed.  After it gains airspeed increase backpressure till it is in a normal climb.