About 20 prominent trees in the lower garden, some of them well over 100 years old, were this week fed 3000 litres of "compost tea" - a brew of water, living organisms and molasses.
The root systems of large trees could extend 40m beyond the trunk and began to intertwine with the roots of their neighbours, tree consultant Frank Buddingh, of Lawrence, said. Over the years, the soil beneath large trees compacted and dried out and the living organisms within the soil died, reducing the flow of nutrients to the root system.
That led to trees becoming stressed, he said, evidenced by poor bud formation in the spring and early leaf shedding in the autumn.
The tea compost would feed the soil and restore the tree to health, he said.
Tea compost was not commonly used in New Zealand yet, Mr Buddingh said.
Manufacturing and applying it in commercial quantities had only been perfected in recent years and he travelled overseas to study composition and manufacture before returning to produce a brew in Otago.
Last year he encouraged the Dunedin City Council to spray compost tea around and on the ailing plane trees in the Octagon, and the treatment was already producing results, he said.
"You can see improvements in one season, if you do it right. The ground under those trees is spongy now, as it should be, not hard."
Mr Budddingh said the trees fed this week might need to be fed again, but soil samples would be taken in a few months before that decision was made.
Eventually, he hoped, all the large trees in the Botanic Garden's lower garden would be fed with the nutrient-rich mixture.
Compost tea and its application costs about $300 per 1000 litres, Botanic Garden collections supervisor Barbara Wheeler said late this week.
"It's cost-effective as far as we are concerned."
Mr Buddingh said the two companies carrying out the project - Omnitree and Sustainable Growing Solutions - had donated 1000 litres of the mix to support the council and the Botanic Garden.
"They are the first local authority and the first botanic garden in the country to try this," Ms Wheeler said.
