You might want to look into using something more permanent like matt spray-varnish or fixer spray (used to 'fix' charcoal drawings) rather than hair spray.
For my own trees I use Woodland Scenic's Tree Armatures
http://woodlandscenics.woodlandscenics.com/show/category/TreeArmaturesFoliages and few 'bottle brush' type railway set trees, also the new Citadel Woods are pretty good and not too bad value for a GW product either.
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~The ravings of a single mad Goblin is bad enough, but such a power-hungry, malice-filled creature as Mortis can never hope to be understood~