I was wondering what the distance is when planting Japanese box hedge. I plan to space them every 20cm as the height of the hedge will be approx 30cm when trimmed.
Is 20cm the correct spacing?
Rather depends what you mean by Japanese box hedge - as far as I'm concerned, it means varieties of Buxus microphylla (as opposed to Buxus sempervirens). Although the varieties differ a little in height and spread, the general rule for planting as a hedge is aroiund 10 cm apart. The link below is to the RHS,where they talk about a much closer planting distance, but the consensus generally appears to be between 8-12 cm - the variation in planting distance is likely down to particular varieties. As your hedge is going to be a low growing one, you need not worry too much about creating a pronounced wedge shape (wider at the base than the top) as it grows - these little dividing hedges are often used as edging around planted areas, and are usually relatively straight from top to bottom.
Ilex crenata? Any other varietal name attached? If I do my metric calculations this is 6 inches by 12 inches high? Way too small for height. I would make the height at least 2 1/2 feet by 2 1/2 feet wide. You are probably looking at 1 gallon baby plants. Plant 1 1/2 feet apart and at least 1 1/2 feet from the edge of the walk.
Dig up the soil and turn over to make a slightly raised bed using a rake and compacting the top of the soil firmly with a chunk of plywood and jumping up and down on it. Have you tested your soil? Make sure the hedge 'goes around corners' to stabilize the hedge in your landscape. You don't want a fast hedge of immature plants. Your sizes might work for a knot garden but not near structures meant for humans. And then the size is iffy for the health of the plant. It is normally much larger, planted in the garden it can't be treated like a Bonsai.
Make a template for hedging. The bottom of the hedge HAS TO BE WIDER than the top by at least 1 1/2" per side. Your hedge is 2' 1/2" high (relax, it will look great for the two years to become a hedge), the bottom of the hedge should be minimum 2' wide, ...so the bottom is 2' or 24 inches; the top width is smaller, 24 minus 3" at 21" (this is critical so that the bottom of your hedge gets as much sun in relation to the top, promoting a thick hedge to the ground).
When hedging imagine yourself in a stiff vice making as little movement as possible. Like a robot. String lines for guides. Sharpen your hedger or your hand shears! Keep your cutting edges on the plane of the sheared sides or top.
Ilex is a super choice for hedges. There are also pillar shaped Ilex that look spectacular, accentuate the verticals in your architecture that will tie your hedge into the landscape and your home.