Spruce have a poor reputation as bonsai. Difficult to style, difficult to encourage the branches to set into new positions through wiring and difficult to create good ramification with little if any prospect of backbudding.
Unfortunately, the ubiquitous Picea albertiana 'conica' has not helped matters; very often picked up by beginners looking for nursery trees to style into bonsai, the Alberta spruce has been known to cause great annoyance and frustration!
However, there are some excellent examples of Spruce bonsai in Europe and Japan; the genus as a whole has some excellent qualities and very often, it is just a question of learning how to work with Spruce and its foibles so that the best can be made of this tree.

This article is primarily based on my experience of Picea albertiana and Picea pungens though these techniques can also be applied with degrees of success on all vigorous Picea/Spruce bonsai.

Spruce vigour

The techniques described in this article assume that your Spruce is vigorous and that care is taken when applying these techniques to weak lower and inner branching.
The easiest way to judge the vigour of a Spruce and its individual branches is to look at the number of new buds that have formed, very often at the tips of the foliage, in late Summer.

Branches with no buds are very weak and are likely to fail/dieback in the following Spring. These are best left alone.
Branches with one bud are weak and will tolerate minor wiring and needle plucking.
Branches with two or more buds are strong to very vigorous and all of the techniques described can be applied.

Timing

Spruce are sensitive to timing. In particular, styling and wiring must not be carried out while the tree is in active growth as branches are very likely to fail and dieback.
In the UK, extensive styling is best carried out from late Summer (August) through to the first frosts around November.

Wiring

Spruce have incredibly supple branches. With the use of thick enough wire and guy wires, branches of over 1" can easily be bent without danger of snapping or cracking. Thinner mature branches can be bent and contorted in a variety of directions to bring the foliage closer to the trunk.
This suppleness is also problematic; wired mature branches can take years to set into their new position, often requiring repeated rewiring. However, I found that on a number of occasions, branches that have had twists and turns put into them have set into position within a year.
When branches of any tree species are wired into a new position, the cambium is broken (however minutely) and the repair of the damaged parts of the cambium layer, by the tree, sets the branch into its new position. With Spruce, it would seem that more manipulation is needed to damage the cambium in order that the tree will be prompted to repair the damage and set the branch.
However, care must be taken as major manipulation of a Spruce branch in the late Spring and Summer will cause it to dieback very quickly. This is why Spruce should be styled from August onwards. After this time I have found that Spruce are very tolerate of all wiring and styling.

Styling

Featuring short needles and very often an overall dense foliage mass the Alberta Spruce seems eminently suitable for bonsai but once the bar branches have been removed, the enthusiast often finds that he is left with a small number of thin, ill-spaced branches that only have foliage growing on the tips.
When considering which branches to keep and those to remove, always value and retain those that have foliage closest to the trunk. Though backbudding can be promoted on Spruce, this will always be limited. Do not rely on any backbudding on the trunk or old branches unless the tree has been very heavily pruned back.

My 5" tall Picea glauca 'Albertiana'

Unless a Spruce has been specifically trained in the ground for bonsai and the branches are appropriate (short with foliage near the trunk), the most suitable design will be one that uses the first 2 or 3 branches with the uppermost (remaining) branch used for the new trunkline and imitates a wild tree or yamadori.
If one studies pictures of good quality Spruce bonsai on the internet or in books, as with Juniper, many of the designs use only a very small number of primary branches (that is branches that grow directly from the trunk) but have had many secondary and tertiary branches developed to greatly increase ramification and foliage density.
Unless you have suitable raw material with the appropriate sized trunk and short branching, informal or formal upright Spruce bonsai are very difficult to style realistically or to any real quality.

Pruning Spruce

Left to their own natural growing pattern, Spruce will produce one flush of growth each year. New shoots extend from buds set in the previous year; these buds are nearly always at the tips of the branches. Backbudding without some kind of pruning regime is very rare.
Without some forward planning and planning, all the needles of the tree will be on the tips of the branches leaving otherwise bare branches.

The Annual Cycle of Pruning

(The exact timing will vary slightly according to your climate and location)

August-October: Needle plucking, wiring and pruning back of this years shoots to a bud if necessary.
To tidy up a foliage pad, new shoots formed in the current year can be pruned back to a bud further back along the shoot. Pruning back leaving no bud for next year will sometimes result in a new bud forming and sometimes will result in dieback of the whole shoot.
As you would a pine, needles can be plucked from the previous years growth, this will occasionally prompt backbudding and makes wiring considerably easier and neater. Do not needle pluck weak shoots. Trees that have been repotted earlier in the season should be observed carefully for vigour.
Needles can either be plucked with fingers or cut with scissors at the base of the green part of the needle leaving the brown sheath that attached the needle to the branch itself.

Small branch of a Picea pungens (Colorado Spruce) From May/June onwards the current seasons growth is easy to identify with all Picea species as it has a lighter more vibrant colour and soft wood. This shoot has only been pruned back in August each year to stop the branch elongating too far. As the May cutting back of new growth has been missed, there has only been one flush of growth each season and the overall foliage density of the shoot is poor.

 

May: Cutting back the first flush of growth
New growth extends from the buds set last year. Allow to extend 1"-2" and then cut the new green shoots back with scissors. On strong apical shoots, cut back by a 1/3, on weaker shoots or those on the lower branches cut back by a ½ or less.
On very weak branches or where a branch is required to extend, do not prune back.

Do not allow the new shoots to extend too far before using this technique or backbudding will be limited.

This image was taken 2 weeks after the new shoots were pruned back (Picea glauca). It is possible to see both the pruned tips of the new growth and the resulting fresh buds starting to extend.

 

May-June/ 2 to 3 weeks later: the second flush of growth follows
In the weeks immediately following new growth pruning, a second flush of buds will develop and start to grow. New buds will develop at the point that you pruned the first flush of growth back, at the base of the first flush of growth and further back on older wood, very often where needles were plucked in the previous August to October.

The second flush of growth will emerge from many points but each new shoot will be much shorter creating much more dense growth. Allow these shoots to extend only pruning back any unwanted individual shoots that grow much stronger than the others.

The tree should then be left until at least August to reinvigorate and develop new buds ready for the following year. The year long cycle is then complete ready for needle plucking to be carried out again.

Compare the density of this Picea glauca branch that has been needle plucked in August and pruned in May of each year, with that of the Picea glauca branch pictured above.