The supply of water must be tailored carefully. Too little water will cause the tree to die but too much water can do the same, as oxygen will no longer be able to reach the roots. The amount of water and frequency of watering must therefore be continuously adjusted according to the conditions. To determine whether the soil is dry and the tree needs water, observing the soil at the surface is not sufficient. Rainfall can make the soil look damp enough, whilst the deeper layer is completely dried out or, in reverse, the top layer may look too dry due to a dry period, whilst the deeper layer is damp enough. In order to determine the actual moisture content, the measurement should be taken at a depth of fifteen centimetres. A rule of thumb to determine watering is that trees with a trunk circumference of less than sixty centimetres need two to three hundred litres of water at a time and trees with a trunk circumference of more than sixty centimetres need three to five hundred litres at a time. Water should be supplied from the moment the buds begin to swell and unfurl. In the case of conifers, it is a good idea to spray water over the crown towards the evening, limiting evaporation by the needles. Checking the moisture levels is important throughout the vegetation period, as is checking the anchoring and development of diseases and pests, so that timely action can be taken.