HELP! Anyone know a safe way to make a really big cloud of white smoke?

It's for a short film I'm making.  In one scene, a large cloud of smoke, simulating a small explosion, is supposed to blossom over some trees.  Does that give you enough to go on?

Thanks.

Perfect. I'd go with the fire-extinguisher for a
one-time effect and shoot MOS, dubbing in the SFX
later. It will give you a good, concentrated blast.

Depending on the height of your trees, you may need
to position someone with the device among the
higher branches. A harness or sturdy platform is
recommended for safety.

A smoke machine will dissipate too quickly
outdoors, and if it's a breezy day, the effect will be
over before it starts.

Also, depending on your local or state laws, you may
be able to purchase a smoke bomb from your
friendly neighborhood fireworks distributor.

Set off the smoke bomb inside a steel drum (or a
METAL -- NOT PLASTIC!) trash can and immediately
contain the smoke with a lid or a tarp.

Once the bomb has fizzled out -- yank the covering off
the can, releasing all the smoke at once. It will
produce a mushroom-like cloud. Whether or not it
will sustain its shape above the treeline is anybody's
guess.

Just be sure to use caution and common sense!
(You might want to keep the fire extinguisher handy,
just in case!)