In the early 1900s, Sir William Ingram employed Wilfred Stalker to collect bird skins in Australia for Collingwood to identify and catalogue at the London Natural History Museum, resulting in his first major publication. In 1907 he collected in Japan and for his work there he was made an Honorary Member of the Ornithological Society of Japan. However his main interest was in the field study of birds; he made the first record of marsh warblers breeding in Kent. He was an accomplished bird artist. His planned book on the birds of France was interrupted by the War and never completed, although part emerged as Birds of the Riviera in 1926. His 1916–18 journals record his war experiences and also his off-duty bird observations, with many sketches made behind the lines in northern France. His published war diaries are packed with his pencil sketches of birds, people and landscapes. He interrogated pilots, including Charles Portal, on the height at which birds fly, resulting in a short paper after the War. He was member of the British Ornithologists' Union for a record 81 years.
After the First World War, horticulture took over from ornithology as Collingwood Ingram's dominant interest. He created his famous garden at The Grange in Benenden and collected plants across the world. His outstanding plant-collecting trips were to Japan in 1926 and South Africa in 1927. By 1926, he was a world authority on Japanese cherries and was asked to address the Cherry Society in Japan on their national tree. It was on this visit that he was shown a painting of a beautiful white cherry, then thought to be extinct in Japan. He recognised it as one he had seen in a moribund state in a Sussex garden, the result of an early introduction from Japan. He had taken cuttings and so was able to re-introduce it to the gardening world as ‘Taihaku’, the name meaning 'Great White Cherry'. His 1948 book Ornamental Cherries is a standard work. In March 2016, a book on his contribution to the survival of Japanese cherries was published in Japan. The author is Naoko Abe, the publisher Iwanami Shoten. An English version with the title Cherry Ingram: the Englishman who saved Japan's Blossoms is out in March 2019 together with the American version The Sakura Obsession. This book recounts the important, almost central, role of cherries in the history and culture of Japan, and describes Ingram's contribution. He introduced many Japanese and other species of cherries to the United Kingdom, as well as his own hybrids. Ingram introduced many new garden plants, including Prunus × incam ‘Okamé’, Rubus × tridel ‘Benenden’ and the Rosemary ‘Benenden Blue’, a natural variant of Rosmarinus officinalis which he collected in Corsica. He also raised many Rhododendron and Cistus hybrids. An avenue of his 'Asano' cherry is one of the features of Kew Gardens. As one of many generous acts, he gave a cherry plant to each of the Walkhurst cottages on Walkhurst Road, Benenden. One of the resulting cherry trees still stands along this road.
