Container Gardening

(Question)

Hi there,
I am filling 5 large planters that are built into my ipe wood deck and am in need of some advice since I’ve never done this before. All the planters are lined with styrofoam and pvc pond liner. I will be planting perennials (not sure which plants yet but nothing bigger or higher than a 4-foot shrub, for example) and a few herbs. I live in Toronto and the planters are in partial shade.
Two of the planters are about 4 feet long by 2 feet wide by 20 inches deep. These each have a 2 inch-wide drainage hole at the bottom of the planter that carries water to the ground through a pvc pipe (These drainage holes are covered with a metal grate and I will also cover them with landscape fabric.
The other 3 planters are 3 feet wide by 2 feet wide by 3 feet deep (quite a bit deeper than the other two) and they rest right on the ground – there’s only a sheet of landscape fabric at the bottom, separating the planter from the ground, and the styrofoam and pond lining are just along the sides.
Since these planters are so large, I want to fill them correctly the first time as I won’t be able to replace all the soil again any time soon (I realize I will have to replenish some of the upper-most soil and add fertilizer on a regular basis).
Also, if it’s not necessary to add materials such as gravel for extra drainage since the planters already have good drainage, I don’t mind skipping that step and filling the planters only with soil; I’m willing to pay the costs to use good soil (or soilless mix, etc.) rather than add unnecessary fillers. It seems that most of the advice I’ve been given is to add some sort of stone or gravel at the bottom, however, but some of the latest information I’ve read indicates that stones may not actually help with drainage.
I have received a lot of conflicting advice but am close to deciding to go with a local nursery that is advising me to add about 1 and ½ inches of stones at the bottom of each planter and then add a mix of 3 parts of a peat-based soil to 4 parts of a triple mix (peat moss, soil, and manure), for a total of 30 bags of the soil and 40 bags of the triple mix (30 L each bag). I would be laying landscape fabric between the stones and soil.
Does this approach sound like a good idea to you? I would love to know whether you would recommend a different approach.
Please see the attached picture if that would be of assistance.
Thanks!

Planter Photo Sept 2015

 

(Answer)

This approach sounds good.  I always add gravel to the bottom of my containers but I first place a larger stone over the drainage hole so that the rest of the gravel doesn’t clog it.  I make sure I’m not blocking the drainage with that stone, just impeding any gravel from falling through it.  I then place landscape fabric on top of the gravel and the soil on top of the fabric.  This prevents the soil from compacting into the gravel and reducing its drainage purpose.  Since you do not wish to replace the soil, I would make sure that the landscape fabric you use does not deteriorate.  Fabric deterioration would eventually defeat the purpose of the fabric.

As far as the soil mixture itself is concerned, I would perhaps recommend a good potting soil rather than a peat moss/triple-mix combination.  I’m worried the triple-mix might be a bit heavy for a planter and hold too much moisture with the added peat moss.  Why not use something that is specifically formulated for planters.  You may have to add more annually but at least you have the correct medium for the containers.  Food for thought.

Hope this helps.