Guerrilla gardening is certainly the 'flavour of the month year decade' and its popularity is growing on an international scale. Hardly a day goes by without some mention of another 'suicide gardener' martyring themselves for the advancement of greening our concrete jungles.

I guess it's no surprise, really! If you consider that hippie tree-huggers have spawned since the 60's it is definitely time for a new generation with a new focus. While tying yourself to a tree in the 'decade that no-one remembers' may have saved a forest, today's guerrilla gardeners are all about creating forests - groves of replenishing greenery.

And they look different too. Our cuddly hippies had no problems sharing their new-found image as Kombi vans, flower decals and copious beads - not to mention the whacked out hair - distinguished them from society's more conservative upstarts. For some, the image was more about the weed than the plants and it eventually became the iconic trademark of drugs, free love and (supposedly!) freedom.

However, the new breed of 'hippies' aren't as extrovert. We've heard about them, seen the odd picture - or two - and maybe embellished a few myths of our own but could we honestly pick them out in a crowd? Would we even be able to identify them in a line-up?

Possibly not.

So what are the marks of a guerrilla gardener? How can we distinguish them from those who would rather see concrete continue to burgeon?

Well....here's a few keys;

• Tiredness - guerrilla gardeners must work under the cover of night, most probably in the wee hours of the morning, in order to not get caught. Daylight hours are a death-trap to them so they prefer to work in darkness
• Piqued Focus - if you travel on public transport with a guerrilla gardener you will notice that as they near the location of their last target they will suddenly become very focused on the scenery outside. As the vehicle continues on a tight smile may even escape their fortified emotions as though success has been achieved
• Dirty fingernails - any gardener knows it's hard to plant quickly with gloves on. Dirt under the fingernails, associated with the first two clues, is a dead certainty that this person is an unheralded ambassador of green-dom.
• Head shaking - clearly a sign of distress, a guerrilla gardener will often shake their heads in bewilderment as they pass abandoned public spaces with limited greenery. If you could gauge their inner emotions you would probably hear the comment, "That's next!"

Fortunately, guerrilla gardening is only illegal if you get caught - so finding clues that will directly associate a gardener with this activity is almost impossible.

What should you do if you spot one? Offer them a knowing glance, a quick wink and a thumbs-up. Then, if you have it pass them a $50 for their next assignment but make sure you do it carefully - you could be arrested for sponsoring acts of terrorism.