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 blueberries 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, it is necessary 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 and bring phosphorus and potassium up to optimum or high levels prior to planting
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 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. Begin modifying the soil at least one year in advance of planting.
To increase organic matter, 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 in contrast to most other crops, which require a higher pH. Sulfur additions to lower pH must 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. Sulfur rates will depend upon soil types and starting pH.
During the year when you are modifying soil, add phosphorus and potassium to bring soil levels to optimum for those two nutrients prior to blueberry planting. Use Potassium Sulfate as the potassium source.
After the 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 of plant loss in harsh winters. When laying out plantings and deciding on 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.
Another 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 the pH is too high for blueberries. Also, composts made with manures as a component may have too high of salt levels and can 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. Blueberries are also very sensitive to weed competition and mulch helps to prevent weed growth. 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 soil pH.
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. Two drip lines per bed, one on either side of the plants, optimizes rooting area, especially in sandy soils. 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. Do not use fertilizers containing chloride (such as KCl – potash).
Place plant orders the year prior to planting. Plants may come as bare root plants, large liners, or potted plants. Large liners and pots have less risk of planting losses. Choose northern high bush varieties recommended for our region. Current recommendations can be found in the Mid-Atlantic Berry Guide. The University of Delaware has conducted trials with additional varieties (many southern highbush types). Contact Emmalea Ernest (firstname.lastname@example.org) for results and additional recommendations.
Blueberries cannot tolerate standing water at any time and site selection is important. Choose well drained sites and consider raising beds or ridges to improve drainage where needed.