You have to be in closed bulb form.  Hit 4 to shift to closed form, then press 1.

Unless you're coming out to nuke you should always be in closed bulb form (better hp/mp regen, little bit more armor).

Also here's a list of the abilities you have and what forms.

Closed-

1- summon shamblers

2- munch shamblers (kill a shambler to regen health and do AE dmg)

3- bramble shield (absorbs damage, can cast on allies or yourself)

Open-

1- thorns thing (can't remember the name, it's a point blank AE, good for clearing creeps, has a debuff that makes enemies take more damage)

2- spike wave (a targetted nuke that delivers a line of spikes hitting everything in its path, also has a debuff I believe)

3- uproot (massive damage over time skill to take down buildings)

For both forms 4 will shift you back and forth.