Locally grown blueberries are a delight, and depending on the varieties planted, can provide a harvest over a long season. At our home in Sunnyvale, we grow primarily Rabbiteye varieties, and also a few Southern Highbush, with picking from June through September. The largest volume of berries come in July, with picking every other day.
We recently were lucky enough to attend an educational event for CRFG members, at Duckworth Family Farm in Sebastopol. Duckworth grows organic Northern Highbush blueberries for several u-pick events over the summer, and Lori Duckworth was generous with both her time and her knowledge. Though her climate and soil are different from much of our area here in Silicon Valley, what she had to share can be helpful for those of us wanting to grow blueberries.
Additional notes below come from Oregon State publications.
Soil
Know your soil type. Blueberry plants have a shallow, mat-like root structure in about the top 12 inches of soil. While much of our native soil here is famously clay based, Duckworth farm is on sandy loam soil with relatively low pH. Either way, Lori suggested testing the soil components, which one can do by sending samples to a lab. Regardless of having soil that is good for blueberries, the Duckworth farm also plants on amended soil, which is mounded 8 to 12 inches for improved drainage and aeration. Good targets for well growing blueberries is to have soilwhich has pH of 4.5 to 5.5, and organic matter of over 4%. Lori mentioned that their goal for organic matter with soil testing, is in the double digits! Duckworth Farms prepares the soil a year in advance with a high percentage of pine shavings (apparently low in pH) incorporated into the soil. Another option might be peat moss. Some reports indicate a beneficial response to well aged compost, with very microbially rich soil allowing a greater tolerance to higher than optimal pH. In our region, the soil pH is quite a bit higher than optimal, and in-ground plantings may need to have extensive preparation with soil sulfur in advance of planting.
Water
Lori Duckworth indicated that she has killed more blueberry plants with overwatering than with underwatering. That said, the plants do want a fair amount of water, but also do not want to be in soggy wet soil. Because the roots are spreading rather than deep, it is important to water on both sides of plants in a row, or in a circle around potted plants, so more of the root zone can receive water. Resources indicate 1.5 to 3 inches of water a week depending on climate and soil. Testing for moisture by digging into the area and feeling the soil is a good strategy. Lori also indicated the importance of acidifying the irrigation water. Watering with high pH water will gradually increase the soil pH to a level which inhibits nutrient uptake by the plants. Most municipal water in our area is over 7 (neutral pH). Her goal for the irrigation water coming out of the drip lines to be pH of 4.2, which is achieved by mixing food grade citric acid powder into the water. Another option for acidifying the irrigation water is to add vinegar.
Mulch
A thick layer of mulch helps with soil temperature and moisture, as well as keeping down the weed growth. At Duckworth farms, they add a fresh layer of pine shavings under the bushes every fall, maintaining several inches of mulch. As the shavings decompose, they add nutrients to the soil, as well as helping the soil pH and organic matter. Other options include chipped wood and sawdust, or leaves, as long as the mulch does not mat and shed water.
Plant Nutrition
After the soil is well prepared and has a year of resting, the Duckworth method is to add 3-0-0 fertilizer at planting time. After the planting, no further fertilizer is needed, according to Lori. Different soils may have different needs of course, and soil pH is very interrelated to the nutrient uptake of the plants. In our soils, generally the nutrient most needed is nitrogen. Too much nitrogen can burn roots, and can also lead to heavy growth of shoots and leaves, rather than energy going into formation of berries. Yellowing of leaves is often a sign that the plant is unable to take up iron because the pH of the soil is too high, rather than insufficient nitrogen.
Planting
Duckworth plants their berries 2.5 feet apart in rows 12 feet apart. Standard recommendations for planting range from 3 to 5 feet, depending on variety and goals. The crown of the plant needs to not be buried below soil level. While some types of blueberries are considered self-fruitful, it is helpful for pollination to have more than one variety of plant. This will also help with creating a longer harvest window. Berry plants themselves can tolerate partial shade, however berries develop better and are more sweet with more hours of sunshine. Duckworth plantings are in full sun.
Pruning
Duckworth keeps their plants compact and under 4 feet in height, in order to facilitate u-pick for families coming to the farm. Lori indicates that plants benefit from heavy pruning to help with stimulation of new growth. Fruit forms on wood grown the prior year, and is best on newer more vigorous wood. Generally plants are pruned when dormant, though in our area, some never seem to go completely dormant. If a plant is not growing vigorously, in spite of good growing conditions, it may be that it needs more vigorous pruning, and removal of all fruit.
Harvest time
At Duckworth Farm, the entire blueberry growing area is covered in half inch netting to prevent access by birds! In home gardens, netting individual bushes is also a solution. Be sure to only net when you have ripening berries, so that the rest of the year birds and bees can help with managing pests, and with pollination. Blueberries can hang on the plant for a long time after they turn blue, increasing in size and sweetness. Lori told us they do not pick until at least 50% of the berries in a cluster are blue, and then pick only the berries that come off with a light touch.
If you get a chance to pick at Duckworth, you are guaranteed tasty berries and a good time!