or at most 4. The unit of pruning is a
fruit cane of 2-1/2 to 3-1/2 feet with its renewal spur. Owing to the
length of the fruit canes they require support and are tied to a high
stake.

This method is used in a large number of vineyards with Sultanina,
Sultana and certain wine grapes, especially Semillon and Cabernet. It
is not to be recommended in any case, as it has several very serious
defects.

The difficulty of obtaining new wood from the renewal spurs is even
greater than in the system shown in Fig. 23 C. The length and vertical
position of the fruit canes cause the main growth and vigor of the
vine to be expended on the highest shoots. The renewal spurs are thus
so shaded that, even though their buds start, the shoots make but a
weak growth. The result is that at the following pruning all the good
new wood is at the top of the fruit canes of the previous year, where
it cannot be utilized. The pruner has to choose then between reverting
to spur pruning and getting no crop or using the weak growth from the
renewal spurs for fruit canes, in which case he may get blossoms but
little or no fruit of any value.

[Illustration: FIG. 24. Forms of head pruning: _A_, vertical fruit
canes and renewal spurs; _B_, bowed fruit canes and renewal spurs.]

Other defects of this method are that the fruiting shoots are
excessively vigorous and therefore often tend to drop their blossoms
without setting and the fruit when produced is massed together so that
it ripens unevenly and is difficult to gather. It also requires a tall
and expensive stake.

Figure 24 B represents an improvement on the last system. It differs
only in the method of treating the fruit canes. These are bent over
in the form of a circle and tied by their middle part to a stake which
may be smaller and lower than that needed for the vertical canes.

This bowing of the canes has several useful effects. The change of
direction moderates the tendency of the vigor of the vine to expend
itself only on the terminal shoots. More shoots therefore are formed
on the fruit canes and as their vigor is somewhat decreased they tend
to be more fruitful. The slight mechanical injury caused by the
bending operates in the same direction.

[Illustration: FIG. 25. Head pruning: fan-shaped head; fruit canes
tied to horizontal trellis.]

The excess of vigor thus being diverted from the fruit canes causes
the renewal spurs to form vigorous shoots, which soon grow above the
f