The urban greening expert said: "We tend to feel that greening our towns and cities must be good, but it is not as simple as that. Indeed, there are benefits, but we mustn't forget the costs such as the use of water and fertilisers."
Pataki said the effects of vegetation on local air temperatures impacts the climate in desirable ways but there are also environmental and economic costs to be taken into account.
For example, fertilised landscapes may emit greenhouse gases such as nitrous oxide. There are very large differences in the way that plants use water, she explained. This affects the way that gardens mitigate flooding, storm water and the urban heat-island effect.
"We need ways to determine whether our landscape designs are effective in providing the services we intended," she said. "There is a great effort to design methods to monitor and validate greenhouse gas emissions but there are few programmes to measure the effectiveness of green spaces in meeting the needs of city dwellers. This needs urgent attention."
Pataki gave a number of examples of work she had carried out that showed a huge variation of results. In some cases the effects were less than expected and in a few cases there was even a net environmental cost.