Tall spindle orchard support systems
Managing High Density Apple Orchards
The reason most tall spindle orchard support systems fail is because the anchorage is too weak, the in-row posts are too far apart, and/or the wires weren’t attached properly. Crop, wind, and snow loads are quite great for tall spindle orchards because all the crop (and/or snow load) is supported on the wires attached to the in-row wood posts and the tall canopy creates a virtual wall that has to withstand strong winds.
The design of the system described below (see Figure 1) is for tree spacing 0.9 m (3 ft); row spacing 3.6 m (12 ft); maximum row length about 150 m (500 ft); top wire height 2.75 m (9 ft); low wire height 0.75 m (2.5 ft); middle wire heights 1.45 m (4.75 ft) and 2.1 m (7 ft). Higher wire heights of 3 m (10 ft) are desirable, but this means taller, more costly in-row posts are needed of 4.3 m (14 ft) or 4.9 m (16 ft). If hail nets are to be installed, in-row posts need to be 4.9 m (16 ft).
- Tieback anchors
Mechanically driven (pounded) tieback anchor posts resist pullout better than screw anchors (used in vineyards) but are more difficult, time-consuming and costly to install. Driven tieback anchors should be at least 13 cm (5 in) in diameter, driven at least 1.2 m (4 ft) deep in undisturbed soil.
If screw anchor tieback anchors are used, they must be at least 1.2– 1.5 m (4-5 ft) long, with at least a 19-22 mm (3/4–7/8 in) diameter rod and with at least one 150 mm (6 in) wide plate at the base to resist pullout and bending. Some suppliers offer a ‘double-helix’ type screw anchors as shown in the sketch. Screw anchors must be installed in undisturbed soil with special bits for skid steer loaders. - End anchor posts
Pressure-treated (CCA) lodgepole pine posts are preferred and they should be driven, or pounded, into undisturbed soil using special equipment. This makes them at least 50% stronger in resisting overturning, compared to augering (not recommended). Every 33% increase in depth doubles these posts’ resistance to overturning. For driven posts, the ‘small’ end of the post goes into the soil first. End anchor posts should be at least 100-125 mm (4-5 in) diameter. Clay soil gives more resistance than sandy soil and dry soils offer more resistance than wet soils. - Geometry of anchoring system
The optimum geometry delivering the most strength is an equilateral triangle (60o at each corner) formed by the end anchor post, support wire connected to the tieback anchor, and ground surface. This is easier said than done because; it is more difficult to pound posts on a angle; the angle means at least 4.3-4.9 m (14-16 ft) posts are needed for end anchor posts in order to get the right height of top wire; and growers don’t want to ‘lose’ more land than need be at end of rows. As shown in the sketch, the top wire must go ‘over the top’ of the end anchor post. This can split the post, so to help prevent this, place a staple (or two) on their sides underneath the wire on top of the post, held in place with staples driven into the top of the post. - High tensile strength wire; Class 3, smooth 12.5 ga; 0.1 in diameter galvanized
High tensile strength wire has more strength per unit cost than low-tensile wire and lasts longer. This wire is tricky to work with as it can spring suddenly, so eye protection and gloves are a must. Do not kink this wire as this weakens it. Carefully roll the wire off the rolls using a Spinning Jenny. Do not lay it on the ground and lift it off. If the wire is not removed from the roll opposite to the way it was rolled on, the wire will behave like a spring and cause you no end of grief. Be very careful when tightening this wire because if it breaks, it recoils with tremendous force. Install the wire first, then hand-tighten it before tying it off. - Wire fastener
Fasteners such as the patented TMGripple join wires and can be retightened if needed with a special tool. There are other fasteners and tightening devices. - In-row support posts
Because of the loads mentioned, in-row support posts should not be much further apart than 10 m (33 ft). These posts are thinner at 75-100 mm (3-4 in) than thicker end anchor posts and they too need to be pounded in at least 0.84 m (2.75 ft). If 3.6 m (12 ft) posts are installed at this depth, this doesn’t allow the top wire to be any higher than 2.75 m (9 ft).
Longer posts are available at 4.3-4.9 m (14-16 ft) but these are much more costly. However if hail nets will be installed, 4.9 m (16 ft) in-row posts can be more justified to support the nets, placed 1.2 m (4 ft) in ground for good anchorage, 3.6 m (12 ft) above ground, and 0.3 m (2 ft) above the 3 m (10 ft) high support wire so there is room for the net to sag between posts without rubbing trees, and to allow hail to shed to the net openings between tree rows. - Leader supports
Leader supports are placed at each tree, but because they do not support weight, simply guiding trees, they do not need to be substantial. There are different types of supports including 12-16 mm (1/2–5/8 in) in diameter bamboo; 20 x 20 mm (¾ x ¾ in) steel angles; or galvanized steel conduit. - Fastener clips
These connect the leaders to the wire so it does not slide along the wire. There are many different types of fasteners including plastic ties, pre-manufactured wire ties, and hand twisted wire ties. - Connecting wire to in-row posts
There are two schools of thought on connecting wires to posts. One is to drill small holes through the posts near the top and feed the wire through to give lots of support both vertically and laterally. The other method is to attach the wires to the outside of the post using staples. Although drilling a hole gives good support, it can be difficult to do in the field and works best if the orchard is relatively flat because the holes have to be predrilled before feeding the wire through. You do not want the wire aligning ‘up and down’ along the row as it might tend to pull posts up, or down, over time.
The other more popular and easier way is to use large, galvanized staples connecting the wire against the predominant windward side of the in-line support posts so the wind pushes the wire against the posts and does not try to work these staples out. Never install staples on the top of a post because the end grain of the wood will not hold them. Use staples that are 50 mm (2 in) long, double-barbed, Class 3 galvanized which have substantially more resistance to pull out than the more commonly used 45 mm (1-3/4 in) long ones. Slash-cut points are better than diamond shaped points because the slash shape forces the staple to ‘rotate’ or screw its legs inside the post away from the flat side as it is driven in. Install two staples for each wire on each in-row post at about 50 mm (2 in) apart. Place the one on the ‘left’ at 11 and 5 o’clock, the one of the right at 1 and 7 o’clock so they form a sort of ‘V’. This prevents the staples from lining up with the grain of the wood which makes the connection weaker, and when the staples are driven in, they rotate to provide a strong connection. Angle the staples slightly ‘up’ or ‘down’ against the angle of pull if there is a rise or dip in the land. Never drive staples home because this damages the wire finish and can kink and weaken it. The wire must be able to freely move under the staple. If possible, leave them with about 6 mm (1/4 in) to go.
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