The gardener does not always have many options when it comes to choosing a site for a dahlia bed. The constraints of lot size, neighbor's trees, buildings, etc., leave very little flexibility. Nevertheless, dahlias prefer partial shade to full sunlight and will grow more robustly with adequate light.
The terms "partial shade" and "full sunlight" are ambiguous. I define "partial shade" as 8 to 11 hours of sunlight (no shading) per day. I define full sunlight as 11 to 14 hours of sunlight per day. As far as light is concerned, dahlias will do just fine if these guidelines can be followed.
While not impossible, making up for sunlight deficiency is impractical for outdoor dahlia beds. But reducing excessive sunlight (shading) is possible and in some cases advisable. For example, if full sunlight in Eugene, Oregon is 1.0, full sunlight in Longmont, Colorado is 1.3. I have no proof to support the size of the difference, but it is a fact that the sunlight in Eugene, Oregon is filtered through 5,000 more feet of atmosphere than the sunlight in Longmont, Colorado. Also, the additional atmosphere is denser than the atmosphere that filters sunlight reaching Longmont Colorado. If this hypothesis is accepted, then selecting a site for a dahlia bed or making a decision about providing shade or not, involves more than just location--altitude is also a variable to be considered.
Shading dahlias is done for at least two reasons. The shading most frequently practiced is to prevent blooms from fading. But it is the second reason that concerns us here. That is to protect the plant from the problems of excessive sunlight--high temperatures, low humidity, and sun scald--and to keep the amount and intensity of sunlight in a range that promotes vigorous growth. Permanent or semi-permanent shading of the dahlia beds can help significantly, particularly if the shading is augmented by a misting system.
An example is shown in the picture at the right. The knitted netting is 30 percent shade cloth and it also serves to protect against moderate hail storms. This particular example uses pulleys and ropes so that the shade material can be rolled up and down at will.
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