The schoolchildren got involved as part of The Sun's school gardening campaign, championed by HW columnist Peter Seabrook.
The design, which shows the RHS logo along with the phrase "100 Years", is made up of around 1,680 plants.
Shona Finnamore, from Stoke Climsland School, said: "The children loved getting their hands dirty in the 'mud pie trays'. They even suggested making their own InstaPlant display for the school as they thought it was 'awesome'. They really didn't want the day to end and it was great to see them so engaged with horticulture."