soil mix for large outdoor planters

karinkelly999March 23, 2008

I am going to plant my very large planters, some 32" wide and tall, and some 36". Some are wooden with no bottoms, others are glazed ceramic. What would be an appropriate soil mix? Some will be placed on a rocky hillside, with morning shade and dappled sunlight in the afternoons. Those I will plant with rhodos, azaleas, and a Japanese Maple. I may put an ivy in one corner of the biggest planter, hoping it will grow over an adjacent arbor. I had thought about just using garden topsoil and peat moss/manure mix, but turns out that was very wrong, according to the garden centre! So, does anyone have a favorite bagged mix that I can buy up here on Vancouver Island? I went to the garden centre today and found that the cost of outdoor planter mix was $6.99 for 20 litres. I would need masses of bags, and doesn't that seem like a lot of money? Also, do I have to fill the entire planters, or can I use broken styro in the bottoms to take up some space?

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For that large of containers, it will be cheaper to make your own unless you have access to wholesale sources. There are scores of different potting soil recipes - seems like everyone has their favorite - but all good ones have in common very good aeration and drainage. That's why garden or topsoil and compost are not recommended. A very common mix is 1 part peat (or coir), 1 part bark fines, 1/2 part perlite, pumice or Turface (high fired clay) and 1/2 part coarse (builder's) sand. For each 1/4 cubic yard, you will need to add 3/4# dolomite lime and some sort of slow release fertilizer, like Osmocote (according to directions). For short term or seasonal plantings you could add some compost or composted manures, but for long term plantings this is not recommended. As it continues to decompose, compost or any other highly active organic material will break down and compact and you will lose aeration and begin to experience drainage problems.

And for long term container plantings, I'd avoid using anything but the potting soil in the containers. Any other material not incorporated into the soil mix will create interface problems, ultimately impacting drainage, and adding any significant quantity of styrofoam or other material will alter watering requirements. Plus, several of the plants you plan to include will appreciate the full root run of all the soil available.

    Bookmark   March 23, 2008 at 10:25PM
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dottyinduncan(z8b coastal BC)

Thanks Gardengal. This is an answer that I also will use having just bought 3 big planters. I went to a talk by Thomas Hobbs at a Garden Show last week, and he told the group that he did not empty his planters yearly -- just added new plants and gave them new fertilizer. When you think of the soil lasting for years, it is certainly worth putting good stuff in the pot to start with. btw, I have a Japanese Maple in a pot that has been growing for 20 - 25 years. I love it. It's on a patio just off the kitchen and I hang cages with bird suet blocks in it in the wintertime and we have many birds visiting us in a place we can see them.

    Bookmark   March 23, 2008 at 10:53PM
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westgate(8b Brit.Col.)

I too have planters with large shrubs and trees, soil never changed but just added to and fertilized. Seems to work well. But as far as large quantities of potting soil go, I had 3 yds of garden topsoil delivered, left it in a covered heap, and used it as needed by mixing in a large amount of peat, (it already had some sand in it) and steer manure with some perlite as well. I have found it works perfectly and it is cheapest to order the full 3 yds (because of delivery costs) and just cover it with a tarp until you need it. Works for me!

    Bookmark   March 24, 2008 at 12:38PM
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novita(SWB.C. z8)

That's exactly what I do, Westgate - delivered soil mixed with peat and perlite. Works fine and the permanent planters just stay planted with a bit of fertilizer each year.

    Bookmark   March 24, 2008 at 12:52PM
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holly_bc(Zone 7B VanIsle)

"A very common mix is 1 part peat (or coir), 1 part bark fines, 1/2 part perlite, pumice or Turface (high fired clay) and 1/2 part coarse (builder's) sand. "

Apologies Gardengal but we have a large chasm created to reach any agreement here. Where is the substance in that? How could one expect any perennial to live long term in this?

Y'all have hit a major pet peeve with me with this subject. Year after year I have seen the 'soil mixes' at nurseries degrade to little more than fluff. There is nothing of substance in any of them. Simply bagged junk presumably in an effort to sell more, more often. I wouldn't even briefly consider planting any long term plant in that stuff.

I'm with Westgate and Novita on this Karen. Get topsoil delivered (preferably with sand pre-mixed) then add some peat & some perlite/vermiculite and some 'vitamins of choice'. Your plants will be most appreciative of real soil rather than the pre-mixed slop avail at most garden centres today. As well you can spend more on plants and less on bags of stuff.

I prefer vermiuclite simply so I don't see the little white stuff in my planter(s) or beds but that may not matter to you.

End rant. :-)))

Sounds like you've a lovely garden planned! Wishing you much success.

    Bookmark   March 27, 2008 at 2:25AM
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