Double cropping primocane fruiting raspberries

Introduction
There has been significant interest from the industry in recent years in double-cropping production systems. This is in reaponse to the observation that the full yield potential of primocane fruiting cultivars is not realised in traditional systems. Fruit is produced bu the upper portion of the cane but buds towards the base of the cane fail to develop. However, if left until the following year, these buds do grow out and produce fruit. In effect the buds towards the base of the cane mimic biennial fruiting cultivars. In double-cropping systems, therefore, both the upper and lower parts of the plant are harvested, the upper part in year one and the lower part in year two.

The work described in this article was carried out a couple of years ago to describe the responses of "Joan Squire" and "Autumn Bliss" to double cropping. The cropping profile of long-cane "Glen Ample" was included as a reference in this experiment, because this is an alternative method for producing a spring crop of raspberries.

Experimental method

Joan Squire in standard (left) and doublecropping (right) systems showing that growth of new primocanes is not affected by double cropping.

Canes of "Joan Squire" and "Autumn Bliss" were supplied with the previous year's fruited wood intact. Two treatments were applied to these canes at this stage.

1. Autumn cropping (standard treatment) The canes which had fruited the previous autumn were cut back to soil level to leave the roots to produce new canes and give the standard autumn crop.

2. Spring + autumn cropping (Double cropped treatment). In this treatment, the canes which had fruited the previous autumn were cut back to the last fruited bud. This left the lower half of the cane to fruit in the spring. In addition to this, the roots were allowed, as in the Standard tratement (1) to produce new canes which would give the autumn crop.

The Spring crop canes were allowed to develop flowers and fruit. In addition, new canes were produced by the root systems but in all tratements only one new cane per plant was allowed to grow and produce a crop in the autumn. When the spring crop ended, the spring crop canes were removed to leave the cutumn crop canes.

Vegetative growth

Autumn Bliss in standard (lef) and double cropping (right) systems showing that growth of new primocanes is delayed by double cropping.

The photographs were taken 6 weeks following planting and show that the growth of the new primocanes was dramatically reduced by the double cropping in "Autumn Bliss" but not in "Joan Squire".

Fruiting Patterns

In both "Autumn Bliss" and "Joan Squire" the double-cropped plants started fruiting at the beginning of May. Fruiting continued until September. However, in the standard tratements plants fruited during July and August.

In "Autumn Bliss" more fruit was produced during July and August by the standard treatment plants than bu those which has been double-cropped. However in "Joan Squire", cropping patterns of plants from the two treatments were similar from July onwards. "Glen Ample" produced a single large flush of fruit from late May to early July.

The total Yields of "Autumn Bliss" in the two systems were very similar suggesting that there is no benefit in double cropping "Autumn Bliss". However, "Joan Squire" produced nearly twise as much fruit when double-cropped than when only the standard autumn crop was harvested. When double cropped "Joan Squire" even produced more fruit than "Glen Ample". Clearly there is potential in double-cropping "Joan Squire".

Fruit size

Of course, the other factor which will determine the success of a cropping system is the fruit size. The timing of cropping and the final yield are important, but if fruit size is severely reduced by double cropping, it will never become a viable production system. The two periods of interest are firstly the spring, when a double-cropped primocane fruiting cultivarcould perhaps replace biennial fruiting cultivars; and secondly the summer and autumn when primocane fruiting cultivars may be the only source of fruit available. Our trials have shown that during the spring, double cropping "Joan Squire" does give larger fruit than Autumn Bliss making it more suitable for this system. However, fruit from "Glen Ample" was larger than either "Joan Squire" or "Autumn Bliss".

During the autumn double cropping reduced fruit size by about 10% in both cultivars. Clearly these are problems that will need to be addressed.

Conclusions
Based on these results there is obviously potential for double-cropping "Joan Squire". However the fruit size of double cropped plants needs to be improved before a double-cropped primocane fruiting cultivar will be able to compete with biennial fruiting cultivars such as "Glen Ample".

Summer 2003