Careful pruning can add value to stands of Sitka
A criticism levelled at Irish Sitka spruce is that timber quality can be lower than desired. Mostly it's a bread-and-butter product, fast-grown and harvested early, and therefore best suited to rough construction and carcassing. Sitka rarely makes joinery grade material, as it suffers from too many dead knots which weaken the timber as well as reduce its visual appeal.
Pruning can make a considerable difference to timber quality, provided it is carried out correctly and at the right time. Some time ago, partly because of the belief that pruned timber should yield a premium price, interest in high pruning Sitka spruce gathered some momentum.
It was also helped by the introduction of a pruning grant, but there was not much take-up. This was partly because of the sawmills not committing to paying a premium, and the grant has now been withdrawn.
One of the main practical difficulties with high pruning is that the ratio of pruned timber to knotty core must be a minimum of 2.5:1 for the operation to add any value.
Given that most of our mills are not geared up to take in particularly large logs, the pruning has to start early in the rotation. A tree with a diameter at breast height of 16cm must be grown to at least 40cm to achieve the ratio and this is roughly the target diameter that the mills require.
In a typical first thinning, a proportion of the stems will already exceed 16cm. So if high pruning is the aim, it must be done with an early first thinning and about 500 potential crop trees per hectare identified for the treatment. High pruning is normally carried out in two 'lifts' -- the first to 3.5m and the second a few years later to 6m. No more than one third of the live crown is removed each time to avoid impairing growth.
While the mills are unlikely to give a guarantee of a premium, owners should still consider high pruning their crops. No mill will turn down a better quality product, which could be your saving grace if you are marketing a small or isolated wood lot.
But pruning is not restricted solely to producing clean, knot-free lengths of quality timber.