Sport and Olympics minister Hugh Robertson, TV gardener Charlie Dimmock, Olympic Gold medal winner Jonathan Edwards, and Olympic Delivery Authority (ODA) Chairman John Armitt are doing the honours to help create around 250 acres of parklands at the east London site.
River banks are being covered with the first of 300,000 Norfolk and Welsh-grown wetland plants that are to form the UK's largest ever urban river and wetland planting.
The first of 4,000 new semi-mature trees are taking root in the Olympic Park with around 100 ash, cherry and hazel trees, grown in Hampshire by Hillier, already planted.
After the 2012 Games, it will be the largest new urban park in the UK for over 100 years.
The new reed beds are being created in a large wetland bowl in the north of the Olympic Park, formerly a 100-year-old landfill site, where spectators can watch the 2012 competitions on live screens.
Robertson said: "The parklands will be a fantastic addition to the Olympic Park, giving people the chance to enjoy all the sporting action in unique and relaxed surroundings and making a visit to London 2012 a day to remember.
"After the Games, this former industrial wasteland will be a wonderful community facility where people inspired by the Olympic and Paralympic Games will be able to go to play sport."
Mr Armitt said: "The parklands will be the centre-piece of the Olympic Park during the Games and are at the heart of the long-term transformation of this part of east London."