Benefits of mulching

alan haighMay 2, 2014

I was just reading the Garden Professors blog and found a study some of you might find interesting. It seems to really bring to light the value, and possibly, eventually, the danger of using mulch for fruit trees.

The danger would apply only to areas where it rains consistently during probably the second half of the growing season in that trees may end up getting more water than what is optimal for highest flavored fruit. It would apply to bearing age trees and not establishing ones.

Here is some of what was reported.

Fertilization had no effect on caliper growth over the two years after transplanting (Fig.1). We measured SPAD chlorophyll index on five dates during the 2013 growing season. Fertilization increase chlorophyll index from 34.0 for the control trees to 35.5. What does this mean? Probably not much. Proportionately this is a very small increase. Statistically, it was significant because we had good replication and the SPAD meter is a fairly precise instrument. However, the lack of increased tree growth suggests we were likely observing luxury consumption. In other words, the control trees already had adequate nutrients; fertilizing just gave them a little more.
Fig. 1 Two-year mean stem caliper growth of London planetrees subjected to root-ball treatments and fertilization.

Fig. 1 Two-year mean stem caliper growth of London planetrees subjected to root-ball treatments and fertilization.

Mulch
Here�s where things get interesting. After two years, mulching increased stem caliper growth of the planetrees by an average of 70% over the trees without mulch (Fig.2). For stats junkies scoring at home, that corresponds to a p-value of 0.001. What�s going on? Well, we know that mulch provides many benefits for trees. The biggest in terms of tree growth is conserving soil moisture. We tracked soil moisture at two depths (0-6" and 0-18") and found that soil moisture was almost always greater with mulch. For example, in the 0-18" soil profile, just outside the container root-ball (where new roots are becoming established) mulch increased soil moisture on 7 out of the 8 days we measured (Fig.3). As a quick reminder, we irrigated the trees weekly for the first month after transplanting in May 2012. After that, they were not irrigated.
Fig. 2 Two-year caliper growth of London planetrees subjected to rootball treatments and mulch. * indicates mean between mulched and non-mulched trees is significant at 0.001.

Fig. 2 Two-year caliper growth of London planetrees subjected to rootball treatments and mulch. * indicates mean between mulched and non-mulched trees is significant at 0.001.
Fig. 3 Mean soil moisture at 0-18" depth for London planetrees with and without mulch. * indicates means for a given date are different at 0.05.

Fig. 3 Mean soil moisture at 0-18â³ depth for London planetrees with and without mulch. * indicates means for a given date are different at 0.05.

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drew51 SE MI Z5b/6a

My problem here is rain during the first part of the season. Once July hits we are super dry. And the transition is part of the problem. The plants are use to heavy moisture, then it becomes dry. I rarely water until July. I also leave off mulch on new plantings till July, when it does help retain moisture.
Not all mulch is equal. I use pine bark and pine straw. Both break down very slowly, so it is useful to retain moisture, and to keep weeds down, but not really a source of nutrition. They just breakdown too slow to be very beneficial. I have bark pieces over 3 years old, and have yet to breakdown,

    Bookmark   May 3, 2014 at 7:47AM
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olpea(zone 6 KS)

Hman,

Thanks for posting. After reading your post, initially I was under the impression the research had been partly conducted with fruit trees. I looked for the blog online and found the the full article. I see now you were applying their research on ornamentals, to fruit.

Mulch does preserve more ground moisture. As we've discussed here before, I think it's the shading (and weed control) which increase soil moisture, as well as perhaps the breakdown of the mulch, which you've pointed out.

I wouldn't necessarily dispute your theory that mulch has the potential to decrease fruit quality in rainier climates. You mentioned you've observed this in your own orchard. I've noticed after very heavy rains (i.e. 2" rains or days and days of consistent 1/2" rains) the peach quality goes down. But I've noticed the same thing from unmulched fruits like strawberries. Incidentally, that's why I will never grow strawberries commercially. They ripen during the rainy season here and don't taste any better than store bought. (We have a small strawberry patch, but just for home use.)

I've noticed the "rain effect" on peach quality is short lived. In other words. It only affects the peaches for about 5-7 days after the rain quits. This is perhaps because the hot days quickly start the peach trees to pull a lot of the excess moisture out of the ground. I'll note that I don't mulch quite to the drip line on mature peach trees (although I do on young trees). This would also have implications on the availability of water to the trees, as many of the feeder roots would extend out past the drip line into the sod, where the ground would naturally dry out faster.

I've noticed the excessive rain affects the most shaded peaches the most. The ones on top of the canopy aren't nearly as affected.

I still maintain some varieties are more affected by rain than others. It's a possible explanation why you don't like Redhaven quality in the East, whereas Redhaven is generally a top quality peach here. Redhaven ripens here, in mid to late July when temps are blazing hot, the sun is high, and the ground is generally bone dry (Another possible explanation is that I read a few months ago that there are numerous "strains" of Redhaven floating around. I wasn't aware of this before. One person's Redhaven may be different than anothers.)

    Bookmark   May 3, 2014 at 8:42AM
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alan haigh

Olpea, the point I was making is that if the mulch increases the growth of establishing trees this much, it is more evidence that it increases the vegetative vigor with established trees the same way- not that anyone disputes this, but the research here (just a single site over 2 seasons, so not really all that definitive) gives an idea of how stimulating it can be and indicates that it is mostly about water retention.

It has been established that deficit irrigation can increase brix content of many types of fruit, so excess water would likely (or surely does) reduce brix

I would expect this all to affect fruit trees in widely different ways even from cultivar to cultivar, as you suggest- but also in your stronger sun it might matter less.

When you talk about rain during ripening, it is not necessarily excess water that is the primary or only problem, I expect it is the blockage of the sun's rays as well.

    Bookmark   May 3, 2014 at 10:50AM
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