Summer shearing of peach trees can reduce fruit size and soluble solids, but if vigorous vegetative shoots that shade the tree interior are removed by hand pruning by early July, quality shoots in the tree interior can be maintained.
Reasons for Summer Pruning
Although the reasons for summer pruning have varied over the years, the primary assumption first stated in the 1880s is that summer pruning promotes reproductive growth (flowering) and dormant pruning promotes vegetative (shoot) growth. Research results from controlled experiments performed during the 1920s and the 1980s did not support this assumption and summer pruning has never been recommended for commercial orchards. Exceptions include removing excess shoot growth to enhance light distribution in the tree to improve red fruit color and to retain fruiting wood in the interior of vigorous peach trees.
In the late 1800s Louis Lorrette described summer pruning for espalier apple trees in France. Dr. H.B. Tukey, in his book "Dwarfed Fruit trees", provided a good description along with figures for "Lorrette pruning". This involved heading shoots to 3 to 5 leaves when the base of the shoot, or the shoot induced by the heading cut, became woody. So an individual shoot may be headed 2 or 3 times during a summer. This repeated heading supposedly would induce flower bud formation on the shoot and the removal of leaves supposedly suppressed vegetative growth the following season by reducing the amount of stored carbohydrates in the tree for spring growth.
The benefits of summer pruning continue to be described on many web sites today. Although it seems that summer pruning should suppress vegetative vigor and promote flowering and fruit quality, the results from a number of studies do not support these claims. In the early 1980s I performed Lorrette pruning on several varieties of apple on M.9 rootstock trained to a five-wire trellis in New Jersey. This practice of repeated heading was very time-consuming and it did not affect flowering, fruit set, of shoot growth the following season. I also visited a couple of home gardeners who followed the Lorrette system without success.
Summer Pruning Vigorous Apple Trees
In the late 1970s, before dwarfing rootstocks were being used commercially, Dr. Utermark, from Germany spoke at the IDFTA conference and described a number of benefits obtained with summer pruning. The benefits included vigor suppression, enhanced flower bud formation, improved fruit size, red color and sugar levels. These claims seemed logical because shoot removal during the summer increased light penetration into the canopy and this should promote flower bud development and enhance fruit quality. This seemed like a perfect topic for my Ph.D. dissertation, and my advisor John Barden agreed, so in 1978 I started my summer pruning research.
While reviewing the pruning literature, I was amazed at how little research-based information existed on summer pruning. There were several articles in trade journals, but no research, so I followed the recommendation outlined by Dr. Utermark. We used 12-year-old vigorous 'Delicious', 'Golden Delicious' and 'Stayman' trees on MM.111 rootstock, which were perfect for studying the effects of pruning on tree vigor. For three consecutive years the trees were pruned in mid-August, about four weeks before 'Delicious' harvest, by cutting all one-year-old shoots to three leaves.
This was very time-consuming, but we thought the process could eventually be mechanized by mowing the sides of the trees. Another set of trees were pruned exactly the same way, but in March.
The trees did not respond as expected. We estimated that we removed about 30% of the foliage in August and this greatly improved light penetration in the canopy interior. Summer pruning also delayed leaf senescence in the fall, so the remaining leaves had higher photosynthetic rates than March-pruned trees. Even the leaves in full sun on the tree periphery had higher photosynthetic rates following summer pruning.
Trunk growth between August and November was suppressed by summer pruning and in greenhouse experiments root growth was drastically reduced by summer pruning. Since trunk and root growth were suppressed after summer pruning, we expected to see a suppression of shoot growth the following season. Although summer pruning suppressed trunk growth each year, shoot growth, flowering and fruit set were never affected. Most years summer pruning enhanced fruit red color development and fruit flesh calcium concentration, and reduced bitter pit, pre-harvest fruit drop, fruit size, flesh starch ratings, water core and fruit soluble solids concentration.
Apparently, we removed enough foliage to reduce trunk growth, fruit growth and sugar levels in the fruit, but not enough to affect shoot growth the following season. In a preliminary experiment I measured total nonstructural carbohydrates (starch plus sugars) in trunk bark and found that carbohydrates were reduced by summer pruning when measured in November but not in April. The lack of difference in spring carbohydrate reserves may explain why vegetative growth was not suppressed the year following treatment.
Another student working with Dr. Barden pruned a different set of apple trees growing in pots every month and measured growth the following year. He found that all trees grew the same regardless of time of pruning. From these experiments we concluded that apple tree growth may vary depending on the type of pruning cuts that are used, but time of pruning will not affect vegetative growth the following year. Since summer pruning was fairly expensive and had few benefits, summer pruning was not widely recommended.
Research results from Massachusetts, Ohio and Europe also supported our results. The only place where summer pruning may be beneficial is when light is the primary factor limiting fruit red color development. In those cases, selective removal of vigorous shoots 3 or 4 weeks before harvest may increase light enough to improve red color without reducing fruit size and quality.
Summer Pruning Peaches
In 1981 I started my career at Rutgers University and many of the peach growers were mowing the tops of their trees with sickle-bar mowers in late-July. I was told that summer mowing peach trees provided many of the benefits that we once thought would occur in apple, but again there were few research data to support these statements. I wanted to verify that peach trees responded differently to summer pruning than apples trees. I worked with several peach varieties of different ages over a 3-year period to compare summer mowing, or summer pruning (a dormant type of pruning in July) with dormant pruning.
The results were very similar to apple: summer pruning or mowing reduced trunk growth, but not shoot growth. Fruit red color was slightly improved by summer mowing, especially for fruit in the top of the canopy and fruit size and soluble solids were reduced. Flowering, fruit set, and the incidence of cytospora canker were not affected by treatment. Summer pruning and mowing delayed leaf abscission and the development of cold hardiness in the early winter, but time of bloom was not affected.
I worked with an agricultural economist to develop a partial budget economic analysis and found that dormant pruning was the most economical way to prune peach trees. Since then I have learned a lot about light requirements in peach and summer pruning can be used to maintain quality shoots in the tree interior if vigorous vegetative shoots that shade the tree interior are removed by early July. Summer pruning later than mid-July will have no effect on flower bud development. Removal of vigorous shoots that create shade can be removed about 14 to 10 days before harvest to increase fruit red color development for some varieties but the increased color will be relatively small.
Summer Pruning Non-Vigorous Apple Trees
During the past 20 years apple orchards have changed tremendously. Trees on dwarfing rootstocks are being planted close together and trunks are supported to at least 10'. The most common training systems include variations of the Vertical Axis and the Tall Spindle. These orchard systems are extremely productive, produce fruit early, produce high quality fruit and require less pruning than large trees on semi-dwarfing rootstocks.
There is interest in mechanizing orchard operations as much as possible, so some growers and researchers have started to mow trees during the summer, primarily to reduce pruning costs. As a horticulturist I hate the idea of mowing because indiscriminant shoot heading typically results in reduced flower spur development, and crow's feet develop on every headed shoot and shade the lower portion of the tree. Dr. Terence Robinson has been summer mowing some Tall Spindle trees and early results look promising.
After mowing, the trees are shaped like an inverted V-shaped narrow hedge. I asked him why he thought tall spindle trees respond so differently to summer mowing than more vigorous free-standing Central Leader trees. Terence said that most of the shoots that are being headed are thin in diameter and lack vigor, so they produce little regrowth during the season that they are headed, sometimes they produce flower buds after being headed and they do not produce vigorous shoot growth the following season.
Before initiating a summer mowing program Dr. Robinson feels that the trees should be dormant pruned well, and then the trees can be mowed for two summers, followed by a detailed dormant pruning in the third year. I think he is only two years into this study, so it will be interesting to see if his 3-year plan works. If his hypothesis is correct, then trees with vigorous shoots will likely not respond favorably to summer mowing, but non-vigorous trees may be mechanically pruned.