Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu and Emmalea Ernest, Associate Scientist – Vegetable Crops; emmalea@udel.edu
Mulching is important for good blueberry growth. Blueberries are shallow rooted and mulch protects blueberry roots from temperature extremes and conserves moisture. As mulch decomposes, it also provides an organic layer where blueberry roots can thrive. Mulch also serves as an important part of the weed control program in blueberries. We have tested six different mulch materials in the blueberry planting at the UD research farm in Georgetown, Delaware in a trial that was planted in 2011.
As stated in previous articles, it is recommended to mulch immediately after planting with a 4 inch layer of organic material. Common materials are aged sawdust, ground yard waste and leaves, ground tree trimmings, ground wood pallets, or bark mulch. Waste hay or straw are also effective mulches. Reapply mulch as necessary, usually annually. In blueberries, it is important to make sure that the mulch that is used does not raise the pH of the soil. Acidic mulches include pine bark and pine needles; whereas, hardwood mulch may raise the soil pH over time. Do not use commercial compost where lime has been added in the composting process to mulch blueberries.
In our trial at Georgetown pine bark, chipped pallet mulch, baled corn stalks and waste hay did not raise the soil pH out of the ideal range for blueberries (4.5-5.0). Composted wood shaving horse bedding and chipped construction waste mulches did raise the pH above 5.0 (Table 1). It is not always clear why a particular mulch would raise the pH. For example, we expected the chipped pallet mulch and the chipped construction waste to perform similarly, but when we tested the pH of the mulch material these two were quite different (Table 1), and had different effects on the soil underneath them. It is a good idea to test the pH of any new mulch materials you plan to use.
Table 1. pH of Mulch Material and Effect of Mulch Material on Soil pH
| Mulch Material | pH of Mulch Material | pH of Soil Under Mulch* | |
| Aged chipped pallets | 5.1 | 4.9 | a |
| Pine bark | 4.9 | 5.0 | a |
| Waste hay | – | 5.0 | a |
| Cornstalks | – | 5.1 | a |
| Aged chipped construction waste | 6.7 | 5.4 | b |
| Aged wood shaving horse bedding | 6.7 | 5.6 | c |
*Mean separation is by Tukey’s HSD. Means followed by the same letter are not significantly different from one another.
The goal with any mulch is to have a material that decomposes slowly and that does not cause problems by tying up nitrogen. You can think of mulch quality as a tale of 3 components: proteins, cellulose, and lignin. Proteins decompose most rapidly of the three and are most common in green materials such as leaves. Because proteins contain nitrogen you do not tie up nitrogen during decomposition. However, high protein mulches do not last as long.
Cellulose is more resistant to decomposition but contains no nitrogen. During the decomposition process, microbes that utilizes the cellulose will scavenge nitrogen from the nearby root zone and can cause nitrogen deficiencies in the crop. High cellulose materials are include sawdust and wood chips. We recommend that these materials go through an “aging” process where they are piled for a year or more to start decomposition process before using in the field.
Lignin is very resistant to decomposition and high lignin materials make the best mulches because of their longevity. This very slow decomposition also limits nitrogen tie up. High lignin materials include softwood tree bark, ground up woody roots, and straw. Conifer bark from mature pine, cypress or other softwood trees contains high amounts of lignin, wax and protected cellulose that resist decay.
When considering mulch longevity; softwoods decompose more quickly than hardwoods; finely textured mulch more quickly than coarse mulch; succulent tissues more quickly than woody materials; and fresh tissue more quickly than dry.
In our trial at Georgetown, the chipped pallets and pine bark mulches had the greatest longevity, the corn stalks and waste hay had moderate longevity and the wood shaving horse bedding and chipped construction waste had the least longevity. Weed growth in the low longevity materials was a problem (Figure 1).
Figure 1. Weed growth in four different mulch materials used in blueberries: a. wood shaving horse bedding, b. baled corn stalks c. waste hay, d. pine bark
Choosing the best mulch material for your blueberry planting will also depend on what is available inexpensively in your area. In southern Delaware some mulch materials can be obtained at low cost or for free (yard waste wood chips, waste straw and hay) while others are pricey (pine bark and pine needles). Also consider the method you plan to use to apply the mulch. Wood and bark chip mulches can be applied with a mulch spreader, whereas hay or straw must be applied by hand.
A final consideration with mulches is the potential for bringing in diseases, such as Phytophthora root rot, with infected woody mulch materials. We experienced this with some of the pine bark mulch we used at Georgetown.