Raspberries are normally grown in rows with drip irrigation. Modern varieties, both maincrop and primocane, are very responsive to irrigation.

Irrigation not only affects the current season's yield, but also the fruit's eating quality, ease of plugging and shelf life.

With maincrop varieties, irrigation can also determine cane growth and yield for the following season. The importance of good watering cannot be emphasised too much.

If the irrigation is poor, discussions of nutrition are purely academic!

Planning:

When planning an irrigation system for raspberries, the grower needs to understand the potential root depth and spread. The ideal irrigation system applies water to the base of the plants and reaches the full depth of the root system. Water going deeper is wasted and also wastes fertiliser. If water does not go deep enough, the deeper roots stop working until the soil water tension is equalised again.

Equipment must be able to apply enough and not too much water in each irrigation cycle. There seems to have been no experimental work carried out on the optimum number of drippers per plant and the ideal lateral spread of water from the drippers. In other words, what is the minimum volume of irrigated soil required to sustain a full crop?

In glasshouse raspberries grown in peat pots, very small (e.g. 3 litres per plant) volumes of substrate will support a full crop. Most soils are not such good rooting media as peat, but a single drip line with emitters at 50cm spacing can grow a good crop. If the water spreads into the alleyway, unwanted spawn outside the crop row is encouraged.

Irrigation Management - some important points:

1. When raspberries are in full leaf, very little rainfall reaches the plant base where the dripper is located. (Even less if the plants are in a plastic-covered, raised bed). But in a dry season, all the roots will be concentrated under the dripper. Therefore, rain may have little effect on the plant's water demand and normal irrigation should be resumed within 24 hours of light to moderate rain, say less than 20mm. Soil moisture sensors under the dripper will show this clearly.

2. Raspberries use most water from the surface, the top 10cm of soil. (see graph2) Therefore, this depth will run out of water first. Raspberries are not able to compensate for drought at 10cm by taking more water from deeper. They will not die if the top 10cm is dry, but they will not grow so well and the fruit will be smaller. Therefore a sensor at 10cm depth under the dripper should be used to determine the frequency and timing of irrigation.

Experience with soil moisture sensors in raspberries shows:

1. Water from a dripline on one side of the plant row will not spread to the other side. Therefore, two driplines per row might be better than one. It has been difficult to detect water spreading further than 25cm from a dripper. In most soils, to achieve a continuous band of wetting, emitters should be at 30cm. See graph below:

Graph of sensors at 20cm depth in glasshouse raspberry crop irrigated with Netafim Ram drippers at 50cm spacing

In this graph, the water use on
6th August was:

10cm 3.3mm 36%
20cm 2.3mm 25%
30cm 1.7mm 19%
50cm 0.6mm 7%

By measuring soil moisture continuously at different depths, it is possible to fine-tune irrigation with precision.

What is the relationship between water use and yield?

The answers to these questions are known for potatoes. They might not be so dramatic for raspberries, but with the high value of the crop, they will impact on profit.

Spawn vs. Fruit. There is often a conflict between fruit size and over-vigorous new cane for the following season. The fruit needs water, but strongly growing spawn will have first call. Many growers' natural reaction is to withhold water as a means of controlling spawn growth. With continuous soil moisture monitoring, it is easy to see the consequences of this action and an alternative method of spawn control must be found.

Tunnels are popular to protect the crop from rain damage and disease and to guarantee supplies for customers by picking in all weathers. Water use under protection is very different and usually much less than in the open. Monitoring is the best way to optimise irrigation for tunnel crops.

Wet Feet. Raspberry roots need oxygen and are very sensitive to waterlogging. Phytophthora root rot is encouraged by maintaining moisture levels high and is spread from plant to plant in free water. The perfect timing for raspberry irrigation is to allow the soil to dry between irrigations to just above the point at which water use slows down. Usually this means keeping a close watch on a sensor at 10cms depth. The crop is then irrigated until water is detected by another sensor at the maximum rooting depth, say 50cm.

Conclusion: There are examples of other issues in irrigation on other crop pages which are equally applicable to raspberries. But the only way for a grower to find out the best way to irrigate his crop, on his soil, with his management system is to monitor for himself. For the good grower who has got most things right, knowledge of the way water behaves in his soil will give the most efficient cost benefit.