When to plant?

The ideal planting times vary per region, but as a general rule you can observe the following;

As soon as the heat is over, usually at the end of September, you can start planting plants until the first night frost arrives.
As soon as the frost is out of the ground, you can resume planting.
In some parts of France, the frost is very low which means that the planting season is long.

It is important that you provide your newly planted plants with sufficient water, even when we feel it is winter!
After a few weeks you will also provide the plants with some fertilizer, if you work this way you can certainly plant your hedge in the period between late September and late May.

If you are using special planting soil, a fertilizer application is not necessary, this is in the soil which also contains the right bacteria, fungi and root stimulators that help the hair roots develop.

A single or double row, what's the difference?

You can plant a hedge in 2 different ways. Single or double row.

Of course you need more plants with a double row, the recommended quantities is one-and-a-half times the quantity of a single row
The choice of planting method is important.
Most people think that you have a thicker hedge with a double row, but that is only partially true.
With a double planting method you do indeed have a thicker hedge, but if you regularly deeply pruned this double hedge is also just a 'slim' specimen.
In principle, even a single row can become as thick as a double if you let it grow slightly with the way you prune.
It is true that you have a thicker hedge with more volume and therefore less looking through.

That is why they say;

If you want to get a closer result right away, choose a double row.
Arn't yiu in a rush time then a single row is an alternative that costs less.
If you lack space then a single row in which you can put more plants per metre / 3 feet is the solution.