Question
We have just bought a home in the lower mainland and are planning to plant a privacy hedge on the west-side of the yard which borders on a busy street. I am thinking of getting Pyramid cedars. We will need build a 2-3 foot retaining wall/box into which we will plant the trees. My question is, what type of fill should we use initially to plant in and how ofter should they be watered after planning (we would be doing this in early December as long as there is no frost). Also, I've heard I should plant them about 2 ft apart and stagger them slightly, do you agree?

Answer
Hi Heather,
Your soil mix should contain 60%-70% of your native soil and the balance should be some type organic matter (compost, leaves, finely ground pine bark, cotton burr compost, manure or a some combination of these).  Pyramidal conifers will do what you want, but there are plants that will give you quicker results, check with your local nursery.  The spacing depends on the variety planted but 2' is probably too close for large growing plants, 4'-5' would be my recommendation and my personal preference is staggered versus straight lines.
Here are a couple of things to think about:
Elevated beds dry quickly, you will have to carefully monitor their water until they are well established.
If you plant too close, the competition between plants for water, nutrients and light usually causes them all to suffer.
Roots will run along the wall, as they increase in size, the wall may be damaged so build a stout wall.
I can't tell you how often to water, there are just too many variables...wind, air temperature, soil makeup, plant size and specie. In an elevated bed, you are not likely to overwater, but very likely to underwater.  There is no substitute for the "finger" test.  Check the soil about knuckle deep.  If it feels moist don't water, it it's dry...water, simple enough.  Make sure you check the rootball of your new plants.  The surrounding soil may be moist, but until the plants are established, all the moisture they get comes from their existing rootball and it will dry very quickly on warm days.
Jim