An understanding of airflow within urban spaces is fundamental to the understanding of both pollution dispersion and heat transfer and therefore highly important in creating healthy, well ventilated and energy efficient urban spaces. London is one of the greenest cities in the world, with many parks, woodlands, over 5 million individual trees. For this reason the impact trees have on the urban climate and more specifically airflow and ventilation is of particular importance to the city and its residents. However the effect that trees have on airflow within the urban environment is currently not well known mainly due to the many variables involved such as tree shape, foliage density, weather conditions and many others. This study focuses on street trees and how they affect the flow patterns and natural ventilation within regular street canyons. A series of Field measurements of wind conditions were carried out in in two streets in South London. One contained small ornamental trees while the other contained much larger mature trees which took up a large fraction of the volume of the street. Extensive measurements of wind speed and direction as well as temperature and humidity were carried out during summer and winter field measurement campaigns. Two 6 metre masts were used in order to obtain vertical profiles within the street. Testing was also done at head height at various points along the length of the street. This allowed us to produce vector plots of the wind velocities to gain information on the type of flow patterns present within the street. The figure below show examples of two of these vector plots taken from a day in which the wind was perpendicular to the street length. Figure 1 shows strong downdrafts on the leeward wall and updrafts on the windward wall indicating the existence of a vortex within the street. The second figure shows the vector plot for street with large trees. There are still downdrafts on the leeward wall and updrafts on the windward wall but the presence of the trees seems to disrupt the formation of the vortex. Significantly lower magnitudes of wind speed and turbulence levels were found for the street containing large trees. Both of which are fundamental in the removal of pollutants from the street indicating that the addition of large trees to a street could inhibit natural ventilation.