Attack units, but not automattically

In my map....well lets just say its not complete yet. Theres this stuff called tangle weed, and basically it hurts you, etc etc, but its almost pointless....I need to make it so that you dont attack it unless you actually click on it and tell the hero to attack it....Cause right now, its defenceless, cause theres 6 heros with ranged weapons. How could I achieve this?
:D

U could make the weed invisable or have shadow meld (if its night) so when a unit goes near it it becomes invisable when it attacks

But wait, if it is invisable, you can still see it right? the units are the ones that dont attack it, correct?

if invisible or shadow melded, the heros wont see it
try making a separate team for the tangleweeds that is hostile towards the heros, but the heros are neutral toward it
this way, the tangleweed will attack the heros, but the heros will need to press 'A' and then click the tangleweed to attack back

What would it do though if I didnt put them in a separate team? Just having the invisable units? Cause i dont really want to make a team called "tangle weeds." It just seems dumb to me... Would it work if they where just invisable? Cause really, I already have them on neutral hostile.