Using Reflective Mulch for Light Management in Orchards
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As high-density plantings mature, there is a tendency to loose colour and sugars on fruit in the lower canopy. The reason for this is that light coming into the orchard is considered to be unidirectional and once it strikes the leaves of the tree, virtually all the high energy short wavelength radiation is absorbed. The top 2/3 of the tree can take the best light.
Under average growing conditions, fruit hanging in the lower canopy are not as likely to get as much usable light as those fruits hanging in the open in the upper parts of the canopy. This condition will hold true even with good pruning and training practices employed. Under average conditions, the calyx end of the apple remains green for many varieties, as light does not come into the orchard at an angle low enough to colour fruit uniformly.
This phenomenon is particularly noticeable with a late maturing cultivar like Fuji. The incoming angle of sunlight and total light energy diminishes significantly after mid August in Southern Ontario if comparing to the June- July time period.
The short wavelength energy at the blue end of the spectrum is responsible for initiating the pigmentation responsible for colour development on cultivars with high light requirements like Gala and Fuji. A direct hit of light is necessary for the process of colouring to take place. This light factor operates in concert with cool temperatures in the fall of the year to result in added crop value.
Light reflecting mulches have now been developed that will redirect sunlight from the orchard floor back up into the tree canopy. When used on selected cultivars in the few weeks prior to harvest value can be added to fruit that would otherwise go to juice. The obvious benefit includes better control over colour development of the crop. With this control, a grower can reduce the number of pickings for multiple pick varieties and harvest a higher % of top quality fruit. This may amount to 1/3 of the entire crop that would otherwise go to juice.
The cost of producing the equivalent volume of fruit by enlarging the planting of a specific cultivar would be many times greater than the investment in enhanced light management practices on established trees.
When used in combination with dormant and summer pruning programs, a grower can gain a huge measure of quality control over the crop. Since the mulch does reflect sunlight in its full spectral distribution, other benefits including enhanced sugar development and better fruit size may be achieved. The qualitative factors other than colour need to be more fully explored.
This product is a metalized plastic with a variety of widths to choose from. Commonly available are widths ranging from 1.0-1.5 m.; this mulch comes in rolls of 600 m to 1200 m depending on manufacturer. A 2000-foot roll will weigh about 13 kg and have 20 grams of aluminum on it. The optical density of the aluminum will be sufficient to redirect sunlight pretty well in its' entirety.
Under average Ontario conditions this product has a usable life of about two months. It should be lifted at the end of the season when the crop is off. The mulch should be perforated to prevent water from ponding. It can be laid by hand or with mulch laying equipment. It should be fixed to the ground by staking or by the use of soil on the edges. The mulch has the added benefit of controlling soil water evaporation and will inhibit weed growth in the tree row.
Laying the Mulch
The mulch can be laid fairly easily with a spooling device fixed to the bucket of a front end loader. The operator backs down the row as the roll of mulch spools out. Soil in the bucket can be scooped out with a spade by hand and placed on the edge of the material as the tractor backs down the row. See Figure 1
Applying the mulch too soon may not provide any advantage. Usually the colour response is quite rapid with noticeable results within a few days after application. Within 3-4 weeks of application significant changes should take place in the lower canopy fruit.
Narrow trees may show a good response with the mulch applied to the West Side only while trees that are broad at the base should have the material applied to both sides of the row.
The mulch has to receive light in order to reflect it. There is no point in putting it down under a dense canopy with excessive shade. This treatment will not correct poor pruning practices nor will it make up for other poor management decisions. For a cultivar like Royal Gala we would suggest a timing of the 3rd week in August. For a late cultivar like Fuji I would suggest late August or the first week of September. It is tough to predict light conditions in late summer early fall.
The following pictures demonstrate the effectiveness of the mulch when used in combination with good dormant and summer pruning programs on Fuji apple. Treatment 1 shows the crop quality resulting from poor pruning practices and no mulch. Fruit appears in the photo in the same position that it grew on the tree. The best coloured is from the top of the tree while the poorest coloured fruit is from the bottom of the tree on the East Side. Treatment 2 shows the effects of a good dormant pruning in combination with a good summer pruning program in the absence of mulch. Treatment 3 is the result from the combination of dormant pruning, summer pruning, and reflective mulch.
Photo Date: October 16, 2000
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