Polish software specialist Tomek Wawrzyczek was pleasantly surprised when he received a 50-centimetre (20-inch) tree seedling in the post.

He promptly planted it in his garden but still has no clue who sent it.

"Who was the kind soul who sent me a seedling? Because I don't know whom to thank," he wrote recently on the micro-blogging site Twitter.

No one has confessed to the unusual mail to date, but from the packaging, Wawrzyczek, 43, learnt it was part of a citizen's initiative in which 61,000 free seedlings were sent across Poland.

Launched by marketing executive Jacek Powalka, 36, the so-called PioSeki online initiative let anyone order up to two free maple, beech, oak or spruce seedlings -- one to keep and one to give a friend.

Powalka's goal was to inspire people to do some good, be it by improving the environment or otherwise, he told AFP.

The idea for the project was first sown on a "beautiful, sunny Saturday morning" in 2007 after Powalka set off to the nursery for flowers but returned with a European variety of sycamore tree seedlings.

"I decided that that year rather than flowers for my balcony, I'd spend the money on trees" for the neighbourhood's overgrown residential square, said Powalka, whose family has a long history of social activism.

He ended up turning the nondescript unused space into a park, making headlines in Poland.

Neighbours helped with planting and chipped in on landscaping plans. The square became a site for community events like summertime starlight film screenings.

-- 'Shame not to spread such a fantastic thing' --

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With the success of the neighbourhood park, Powalka thought it would be "a shame to not spread such a fantastic thing to all of Poland," so he took tree-planting to a national level this May.

Through the PioSeki campaign, Powalka strove to nudge Poles into improving their communities -- such as sprucing up areas too small to interest local officials -- rather than relying on the government.

"There are fragments like a sidewalk curb with a plot of grass: it's a place that will always just be a piece of sidewalk but it could have been a beautiful spot for a tree," Powalka told AFP.

The send-a-seedling campaign cost a total of 560,000 zloty (130,000 euros, $160,000) and was sponsored by several companies, including Powalka's own postal-service employer and co-organised by Polish public radio.

It drew interest not only from individual Poles but also schools, hospitals and municipalities, even prompting nurseries and national parks to offer to donate seedlings for round two.

And London's large Polish community has asked to take part next time. While the project began as a one-off, Powalka is mulling over plans for a possible sequel next season.

Greenpeace Poland director Maciej Muskat described the campaign as a breath of fresh air.

"The fact that this wasn't just a matter of sending out emails or signing petitions, which is sort of the standard, but it was tied rather to planting actual trees is a kind of novelty, and I strongly applaud the act," he said.

Krakow resident Jakub Cholewka, a 30-year-old translator who ordered a beech for himself and a maple for his wife, believed the initiative had the potential to prod Poles into planting more seedlings.

"I certainly won't stop at the trees I got through the campaign," he said.