Irrigation and nutrient management for raspberries

Irrigation

Growers must use irrigation on raspberries, especially during the fruiting period, to get high-quality fruit. Water stress causes unmarketable, crumbly berries, and reduces plant growth and yield.

Choose between drip irrigation and overhead sprinkler systems. 

• Drip irrigation is preferable for raspberries because it does not promote leaf disease and can also be used for fertigation. It is required for high tunnel raspberries.
• Overhead irrigation is strongly discouraged for raspberries. Overhead systems are less water efficient and increase foliar diseases because much of the water hits the plant canopy before reaching the soil. 

Establish a water source and test the water quality prior to selecting a site for your raspberry planting, to determine whether the water source is suitable for raspberries. Water that is high in certain macronutrients can alter the soil pH and limit the availability of other micronutrients. Test the irrigation water regularly and keep the soil pH between 6.0 and 6.5 to minimize nutrient management challenges. 

High tunnel raspberries have greater water needs than open field raspberries because they grow larger and produce more fruit, and because of the elevated temperature in high tunnels. 

Water needs also vary throughout the growing season. Be sure to keep the plants well-watered between the months of July and August, and during the fruiting period. Raspberry plants transpire up to a quarter of an inch of water each day from June through August. 

Installing drip irrigation

Drip, also called trickle, irrigation systems deliver water uniformly and only to the raspberry plants. The slower release and the lack of unnecessary watering between rows reduces water lost via evapotranspiration. Soil type, soil water-holding capacity, and maturity of the raspberry crop determine what the irrigation schedule may look like in a given year. 

Install two lines of drip tape per raspberry row. Evenly space the two tubes down the row width to ensure all roots and fruiting canes have access to moisture as they spread. A more permanent line system is generally more favorable than disposable ones as they tend to be less susceptible to rodent and insect damage.

A new planting in sandy loam soil requires 18 gallons per day per 100 feet of row. A mature planting in the same soil type requires 27 gallons per day per 100 feet of row. Increase the water rate for sandier soil, and decrease it for heavier soil.

Irrigate in frequent small amounts to keep up with increased water demand in the peak evapotranspiration months of July and August. You may need to increase weekly water application in peak summer heat.

For more information on how to set up a drip irrigation system, see Irrigation strategies for vegetables.

Nutrient management

Exact fertilizer needs differ between farms, as they depend on the age of the planting and the type of raspberries (summer-bearing vs. fall-bearing), soil qualities, and environment (open field vs. high tunnel).

Determine nutrient requirements through soil nutrient and foliar tissue analysis. 

• Conduct a soil nutrient test before planting, and every three years thereafter. 
• Conduct a plant tissue analysis once raspberries reach the second year of production, and on alternate years thereafter. 

Soil nutrient testing

Soil nutrient tests are widely used to measure the level of nutrients present in the soil. Fertilizer recommendations are generated based on research-based optimal nutrient ranges for the crop being grown. The Nutrient Management Guide for Fruit and Vegetable Crops lists the optimal ranges for each macro- and micronutrient for major specialty crops including raspberries.

Sandier soils found in some parts of Minnesota may have potassium deficiency and higher nutrient leaching.

The ideal pH range for raspberries is 5.5-7.0. More acidic soils may require amending with lime to raise the soil pH. Compost with a high pH can also be used to increase soil pH and the organic matter content of the soil. Soils with a pH above 7.0 should be amended with sulfur prior to planting.

Testing the soil 6-12 months before raspberry planting allows enough time to adjust nutrient levels and soil pH if needed. Recommendations are described in the nutrient management guide.

For more information on soil testing, visit the soil testing laboratory.

Foliar analysis

Plant tissue (foliar) analysis measures the actual nutrient concentrations in the plants and compares them to optimal ranges, thus providing a more accurate depiction of nutrient availability for raspberry production. For example, the micronutrient boron, which is critical for bud break and fruit set, is better predicted with foliar tests than soil tests.

Use foliar analysis in raspberry production in two ways: to diagnose current season nutrient issues, and to optimize annual fertilizer programs. 

For routine monitoring, sample young, fully expanded leaves at the same time every year. Normal foliar analysis is done midsummer, when nutrient concentrations are the most stable.

If nutrient deficiencies are suspected, you may collect samples at any time. Submit a second sample from healthy plants, for comparison.

Collect at least 50 newly expanded primocane leaves, selecting only one leaf per primocane. The leaves should be free of disease or insect damage. 

Contact your local soil analysis laboratory and follow their plant tissue collection and preparation guidelines.

Fertilization

Fertilizer programs are most cost-effective, sustainable, and efficient when based on soil and foliar tests and objective research. 

Fertilizer can be delivered by granular broadcast or through drip irrigation. Many growers choose to apply nutrients through their drip irrigation (this is called fertigation) to save time and ensure even fertilization rates to their plants.

Fertigation may include a mix of macro- and micro-nutrients. See the table below for nitrogen recommendations. Soils with higher sand content typically require higher levels of potassium (K). 

It is possible to over-fertigate. This means that excess nutrients are applied that offer little to no benefit, or when levels of certain nutrients become toxic to plant growth. Excess nitrogen, for example, leads to increased vegetative growth and a reduction of fruiting laterals due to longer internode spacing. It can even cause leaf burn symptoms. While we occasionally see claims about the benefits of routine calcium application to berries throughout the season, peer-reviewed research involving extensive in-field trials generally have not supported routine calcium application. Soil calcium levels are generally high in much of Minnesota.

There are many products on the market that claim to increase plant vigor, yield and fruit quality. Use research-based information whenever possible, and avoid products that do not provide a strong research basis to support their claims.