Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
There has been an increase in interest in growing blueberries in Delaware and we have had several new plantings in Delaware in recent years.
Blueberries are very specific in the type of soil conditions in which they will grow. The ideal blueberry soil will be sandy but with high levels of organic matter, it will have a pH between 4.5 and 5.0, it will be well drained in the surface soil but will ample subsurface water. These are the conditions of southern New Jersey where they are native and where there are large commercial plantings. We only have a small area of Delaware with those characteristics – the “black soils” were marshes were drained in southern Sussex County. In all other areas of Delaware we need to recreate those conditions.
There are five keys to success with blueberries:
1) Increase soil organic matter before planting
2) Drop soil pH to between 4.5 and 5.0
3) Put organic material in the planting hole during planting
4) Mulch the plants well after planting
5) Install a drip irrigation system
The following are some more details on each of these keys.
A common mistake that is made is to plant blueberries before the soil has been modified. Normal agricultural soils will have a pH around 6.0 and organic matter below 2%. Blueberries will not grow well in these conditions. It is advised to plan at least one year in advance to modify the soil.
In order to increase organic matter you should plant cover crops and consider amending the soil with additional organic sources such as pine bark fines. Do not use composts that have high pH.
The pH of the soil will need to be modified. This is done by adding elemental sulfur at recommended rates according to soil type and the amount of pH drop required. Again the target is between 4.5 and 5.0. Blueberries are among a group of unique plants that are acid loving. This is in contrast to most other crops that require a higher pH. Sulfur additions need to be done the year before planting. This is because bacteria in the soil need to react with the sulfur to form an acid that lowers the pH. This only occurs when soil temperatures are warm and it takes several months for the full reaction to take place. You cannot apply sulfur in the year of planting and expect the soil pH to be in the acceptable range for good first year growth.
During the year when you are modifying soil, add phosphorus and potassium to bring soil levels so that they are optimum for those two nutrients prior to blueberry planting.
Once soil has been properly modified, you can plant the blueberries. This is normally done in the spring. Fall plantings are possible but there are higher risks to losses in harsh winters. When laying out your planting and deciding on your between row spacing, think about how you will apply mulch and pesticides and whether you will be using netting to exclude birds. Rows will need to be wider if large equipment is used for mulching or spraying, but wide row spacing will increase costs if netting is needed to prevent bird damage.
A key to planting blueberries is to add organic matter to the planting hole. The most common practice for smaller plantings is to use one gallon of moistened peat moss in each hole. Other organic materials can be substituted but they should be low in pH and should be at least partially decomposed. Most commercial composts are not acceptable because lime is added during the composting and the pH is too high for blueberries. Also, composts made with manures as a component will have too high of salt levels and will injure the blueberry roots.
After planting, blueberries should be mulched heavily. Blueberry roots are shallow and need to be protected from high soil temperatures. In addition, the mulch will conserve soil moisture and provide additional organic matter as it slowly decomposes. The best mulch materials are high in lignin and acidic in nature. Pine bark is ideal but is often not readily available. Aged wood chips or ground yard waste that has been aged makes good mulch. Sawdust must be partially decomposed before use to avoid nitrogen deficiencies. Avoid mulches that increase pH.
For most plantings, drip irrigation is recommended for blueberries and is best placed under the mulch. Because blueberries are shallow rooted, frequent irrigations during our hot summers will be needed to get the plants established and growing well. Overhead irrigation can also work if designed properly.
Do not put fertilizers in the plant hole and avoid adding any fertilizer until plants are established. In the first year, blueberries will need about 20 pounds of nitrogen and nitrogen should be in the form of ammonium sulfate or urea. Do not use N sources that contain nitrate.
