They are found growing in dense copses, each tree within reach of several of its peers. For the most part, the trees are dormant. They remain inanimate, content to simply grow and photosynthesize. Only when one of their number is threatened - perhaps by woodcutters or dangerous beasts - do they stir into motion. At this point, the whole grove goes berserk, lashing at intruders with branches and roots.
An encounter with Trees That Bleed is all about positioning. By the time the trees animate, the PCs are probably in their midst. Draw a map of the grove, put the PCs somewhere in the middle. Each tree ought to be within 10 feet of one or more of its peers. Each tree has 15 feet reach with its branches and 5 feet with its roots. With the large numbers of trees present, the skill in the fight is to find points where fewer trees can reach you: the gaps and clearings.
As plants, Trees That Bleed are affected by spells like 'command plants' and 'talk to plants' but not by spells like 'hold monster'. Their tough, woody nature means that they take a maximum of 1 damage from everything except Axes & Fire.