Shade within orchard trees can arise from self-shading or from adjacent trees or shelterbelts. Light levels in midsummer within well-spaced, traditionally pruned large peach trees can drop to 4–15% of incoming radiation. Sour cherry trees tend to be more dense, and light levels as low as 2–4% have been reported. Vigorous grape canopies can be very dense, with irradiance within the canopy reduced to less than 1% of ambient. With such dense canopies, between-row shading will be unacceptable if the ratio between canopy height and clear alleyway spacing exceeds 1:1 (Smart 1989). With the much more open canopies of apples, such ratios may be less useful, as acceptable light distribution within the trees can be obtained over a wide range of ratios of canopy height to clear alleyway spacing. With apple canopies, a ratio between actual light interception and $F_\mathrm{max}$ may be more useful. If this ratio exceeds 0.7 then self-shading is probably excessive.