Straw as a reflective material

Summary

We wanted to see if it was possible to harness the light reflective properties of straw (Blanke 2008) to increase the incidence of red blush on Ambrosia apples growing on a north-facing slope. In a controlled experiment carried out in 2020, we found that putting down straw was not powerful enough to overcome the light deficit on our slope. We also found that covering the grass alley with straw had negative effects on vegetation growing in the alley.

The problem

Ambrosia is a bi-coloured apple with a golden background and a red blush. In recent years, retailers have been demanding a uniformly high proportion of red blush on this variety (allegedly to satisfy consumer preferences), and this has put intense pressure on farmers to grow redder Ambrosia. This demand forces significant costs onto Ambrosia growers at a time when apple prices are at an historical low, leading many growers to give up on growing this variety.

On our orchard, there is one area where our Ambrosia consistently struggle to obtain a uniform red blush, and that is on a north-facing slope near the farmhouse. Direct exposure to light is the main stimulant of red colour in Ambrosia, and the trees growing on this hill, especially at its bottom, receive fewer hours of direct sunlight per day than trees elsewhere. One way to increase red blush on Ambrosia growing on this north-facing slope, then, would be to increase the amount of light trees on the hill are exposed to on a daily basis.

One way to increase the overall amount of light in an area is to put reflective fabrics down on the grass alley. Unfortunately, reflective fabrics are very expensive (about $4,000/acre), are labour-intensive to install, and demand perpetual reinvestment as they degrade into gigantic sheets of garbage in only five to six years. From an organic perspective, reflective fabrics are almost certainly disruptive to the complex biotic communities we are trying to foster in our grass alleys. In any case, reflective fabrics are not intended for installation on slopes, making them a non-option for our slope.

In a controlled study, Blanke (2008) compares one popular variety of reflective fabric (ExtendayTM) with two alternative reflective materials: a biodegradable white paint, and a layer of fresh straw. According to Blanke (2008: 224), freshly applied straw is equivalent to ExtendayTM in the amount of light it reflects. Could laying down a layer of straw be the solution to our problem, a means to bring more light to our north-facing slope and more red colour to the Ambrosia apples growing there?

According to Blanke (ibid.), the problem with using straw as a reflective material is that unlike ExtendayTM, its reflective properties degrade over the season as straw encounters the elements. We wonder: if straw were re-applied as needed, could its reflective properties be sustained?

Blanke (2008) also notes that straw, being a high-carbon soil amendment, might have unintended effects within an orchard soil system. We wonder if removing the straw layer at the end of the season could mitigate these effects. We also note that in our organic system, this removed layer of straw could serve as a valuable input to our compost pile.

The experiment

Six grass alleys on our north-facing slope were included in the experiment (we designate this the “experimental zone”). Approximately 4 weeks prior to the beginning of Ambrosia harvest, we covered three alleys in a 2-4 inch thick layer of organic flax straw, while leaving three alleys as grass controls. Alleys assigned to the experimental and control conditions were alternated.

Immediately after putting down straw, the difference between experimental and control alleys was visually striking.

Two weeks later, we reapplied straw in the experimental alleys. By this time, there were some blades of grass and dandelions sparsely poking through the straw. Since the straw we had previously applied was only partially degraded (darkened in colour), we used much less straw for this second application.

The day before our Ambrosia harvest began, we went out and measured the incidence of red colour achieved on our north-facing slope. Specifically, we measured the proportion of apples visible on each side of each tree which had obtained enough red colour to qualify them as Extra-Fancy (30% Red No. 4).

In addition to our experimental vs. control condition, we treated east vs. west side of the tree as a separate factor in our analysis. The reason for this is that our tree rows run north-south, and apples on the west-facing side of trees, which are exposed to the afternoon sun, tend to mature faster than apples on the east-side of trees. We wanted to factor in the possibility that apples on the west side of trees might obtain more red by the time of measurement. Thus, every tree in the experimental area was measured from two angles, each time measuring the proportion of apples on the tree with red blush that were visible when standing about 2m back from and parallel to the tree row.

In total, 30 measurements were made within each alley (15 east-facing trees, 15 west-facing trees), with 180 measurements made in total.

Unfortunately, the timing of measurement was not ideal. Since the Ambrosia trees in our experimental zone are always the last Ambrosia on the farm to mature, it would have been better to measure their red colour just prior to going over them for the first time (about three days later than the day they were in fact measured). Because we were busy with the Ambrosia harvest, we just did not have enough daylight hours in which to do this. Therefore, the measured effect of straw on the development of red colour in these Ambrosia is almost certainly an underestimate, though how much of one is unclear.

Results

Since our experiment was set up as a 2x2 design, we initially carried out a two-way ANOVA, with experimental vs. control and east-facing side of tree vs. west-facing side of tree as our independent variables. No significant main effects or interactions were found in this first analysis (p = 0.05).

We then carried out a three-way ANOVA, in which we also considered each tree’s position on the hill, with bottom of hill vs. middle of hill vs. top of hill as our third independent variable. (Thanks to Emily Hindalong for carrying out this analysis for us!) With p = 0.05, the only significant result we obtained was a main effect of position on the hill. More specifically, we found that trees growing on the bottom third of the hill obtained significantly less red colour than trees growing on the top two thirds of the hill. Given that the bottom of the hill is its most shaded portion, this result is unsurprising.

Discussion

Though fresh straw has been shown to be as light reflective as ExtendayTM (Blanke 2008), and while straw’s reflective ability was visually obvious to us, its application to the grass alley was not effective at increasing the incidence of red blush in Ambrosia apples on our north-facing slope.

We suspect that the straw layer did increase the amount of reflected light present on the hillside, but that this was simply not enough to overcome the light deficit inherent to our north-facing slope. We wonder whether even ExtendayTM, with its more consistent reflective properties, could overcome the light deficit at the bottom of a shaded hill like ours.

It is possible that we might have found an effect if we had measured red colour at a more appropriate time. Red blush on Ambrosia develops very fast, and three days would have made a difference to our measured outcome. Here, we encounter a limitation inherent to on-farm research, which is that we could not prioritize the research project over the more pressing need to harvest our Ambrosia.

All things considered, the experiment cost us about $300 in materials and labour, or less if you consider that the straw was later repurposed as an input to our compost pile (more on this below). We are glad that we did the experiment, but we are not convinced that it is worth repeating.

If the market for Ambrosia continues on its current trajectory, transforming Ambrosia into a predominantly red apple at significant cost to the farmer, we will have to give up on growing this variety on our north-facing slope, and indeed on north-facing slopes throughout the valley.

Compost and beyond

After our Ambrosia harvest, we collected the straw we had put down in our experimental alleys and repurposed it as an input to our compost pile. By this late point in the season, we were happy to see that the straw was already half-composted, filled with worms, fungi, millipedes, and smelly microbes - a great head start to next season’s pile.

However, in practice, it was not possible to remove all of the straw. A thin layer of partially-composted gunk remained on the experimental alleys.

The downside of applying straw became apparent in the following season (spring 2021), when vegetation was slow to regrow on those alleys where straw had been applied. Apparently our straw layer, which was in place for about two months, had smothered the plant communities in these alleys, which then required a long recovery period extending well into the next season and beyond. We therefore echo Blanke (2008) in advising caution to anyone considering putting down straw for use as a reflective mulch, as it did indeed have unintended effects on the vegetation, and hence the soil, in our alleys.

By Katie Sardinha

References

Blanke, M. M. (2008). Alternatives to reflective mulch cloth (ExtendayTM) for apple under hail net? Scientia Horticulturae 116, 223-226.