The sprawling grounds of the Istana were of particular interest and affection for founding Prime Minister Lee Kuan Yew, and where his vision of a City in a Garden first took root.

Yesterday, President Tony Tan Keng Yam and his wife, Mrs Mary Tan, planted a young tembusu tree on those grounds to mark the first anniversary of Mr Lee's death on March 23 last year.

Dr Tan paid tribute to Mr Lee's contribution to the Istana gardens and beyond, saying: "Making Singapore a City in a Garden was his great vision and foresight.

"And Mr Lee pursued this initiative with his usual tenacity, meticulousness and persistence."

The tembusu was one of Mr Lee's favourite trees as it is known for its strength and robustness, said senior curator Wong Tuan Wah, who worked closely with Mr Lee on various garden projects for 19 years.

Mr Lee took a liking to a particularly majestic 150-year-old tembusu tree on the Istana grounds and noted how it lived through many important events in Singapore, said Mr Wong.

Remembered by many as the Istana's chief gardener, Mr Lee would also use the grounds as a testing bed for new flora and fauna that he encountered around the world.

If successful, he would later introduce them in the Botanic Gardens and parks throughout Singapore.

An example is the foxtail palm, a name inspired by bushy fronds that look like the tail of a fox. Mr Lee had come across the palm while on a visit to Australia and asked for it to be planted in the Istana. "Now, they have become a very attractive feature of the Istana gardens, beside the swan pond," said Dr Tan.

Constructed in July 1968, the swan pond is the largest of five ponds on the Istana's 40ha grounds.

It is also home to a pair of mute swans, which both Mr and Mrs Lee would visit and feed during their evening walks. Over the years, other birds such as whistling ducks and magpie robins were also introduced.

Mr Wong said at Mr Lee's request, chemical fogging of the grounds was not carried out as he feared it would kill the insects the birds feed on.

Mr Lee would also often display a glimpse of his romantic nature during the evening walks, he added.

He would sometimes pick a cluster of fragrant white flowers, more commonly known as the breadflower. A favourite of Mrs Lee's, the flowers have a sweet pandan fragrance.

He also started the country's tree-planting initiative, first in his Tanjong Pagar constituency.

"It was a symbolic reminder to Singaporeans of the importance of making sure that Singapore is not a concrete jungle, but a liveable city.

"The best way to remember him is to keep Singapore going for the next 50, 100 years," said Dr Tan.