Alkaline, clay soil is not suitable for azaleas/rhododendrons so the planter box is a good idea.

3' x 18" x ? should be large enough depending on the particular azalea variety and, especially, the missing dimension.

Azaleas and rhododendrons need an acid, highly organic planting medium. If you can buy bagged material labeled as suitable for ericaceous plants, that's one possibility. I'd avoid anything labeled as "topsoil" by itself. Topsoil is an imprecise term and is often not very good for any kind of plant. At the least, make sure what you use is 50% or more organic content. Partially decayed softwood bark, conifer needles, whatever else might be available locally can be used to make a suitable medium.

Some potential problems with containers: You'll have to water more frequently than you would with plants in the ground. Azaleas hate and can die from excessive heat in the root zone, so avoding too much direct sun is important. Warm and wet planting medium increases the chances that various root diseases will flourish.

Thanks for the help, mainegrower. Sorry about the missing dimension but I believe 12' is what I have measured out. Basically it will go down the length of a small shed in my backyard. So it will be 12' long, 3' wide and 18" deep.

I have read about pine needles and leaf mold but I dont have a clue where to buy things like this. I havent seen it at the big box stores (although I may have overlooked it) and the place where I usually buy soil and mulch doesnt carry it either so I am sort of at a loss as to where to get these things.

They do sell cedar mulch, pine bark and 'compost'. I dont know what goes into the compost but they sell it. Would any of these work for my organic content? Or any other suggestions on where to buy what I do need?

Thanks again for the help!

If they sell bulk pine bark mulch, chances are excellent that some of it at least is just what you want. The old black stuff has aged enough to be an excellent planting medium all by itself if you're willing to check it for possible fertilization needs.Perhaps they could also deliver a mixture of bark and compost - say 70% bark, 30% compost. That would be a whole lot cheaper than anything found in bags, too. To fill the planter you propose, you'll need just about 2 yards - that's a huge number of 2 cubic foot bags!

Great! I will look into that mixture but just to be clear, I wouldnt need the topsoil at all if I go with a mixture of bark and compost?

Thanks!

btw here is a link to their website and you can see their products and the ones I am talking about.

Here is a link that might be useful: Mulch

Sorry for posting again but I checked out their western mix on their site. How does that sound? It says it is a mix of topsoil, compost and pine fines.

Thanks again, maine. :)

I think of a planter box as a structure that separates the stuff inside from the 'real' soil. Is that what you are building? Or are you wanting to build a raised bed?

The medium suggested above will not be a permanent solution and will behave much like any other container mix: it will break down into smaller and smaller particles and require change outs periodically. Just like with any other container. I'll be that's not what you had in mind.

But knowing if your 'planter box' is going to have a bottom on it or be sitting on the actual outside soil is important to know.

I had planned to build the box without a bottom so that it could drain freely over the existing soil but I could go either way depending on what provides the best growing situation for the bushes. The soil in this area is highly compacted and the only thing that grows there is Japanese Clover. Even the Crabgrass has avoided this area so it must be bad. :P

Re "western mix". It sounds suitable, but no way of knowing for sure without having the proportion of the three ingredients and, especially, the mixture's pH - this should be acid enough for azaleas and rhododendrons. Remember that you would want a higher proportion of bark than someone who's using the mix for another purpose.

The company has a very nicely detailed and informative website, so I wouldn't hesitate to ask them what they'd recommend.

rhizo 1 is correct about the breakdown of material in a container (it happens in a raised bed, too), but in a container of this size, it will not happen very quickly and maintaining surface mulch will help. Just be sure the azaleas's roots don't become too deeply buried.

Placing the container directly on the ground is probably the best way to go. Many people would also suggest using heavy duty landscape fabric between the container and the native soil. This barrier is generally designed to prevent earthworms from mixing the undesireable lower native soil with the container mix. Can't hurt to use the fabric, but possibly unnecessary in your situation.

Thanks for the help everyone. I will contact them and find out the proportions in their western mix and then get extra bark if necessary. :)

Hi Everyone,

I have had a Rhododendron plant for 6 years. It bloomed the first year but has never bloomed or grown much since. I add acid fertilizer and nitrogen and plant has grown to 2 feet tall with limited leafing and no blooms. It looks healthy but not spreading or blooming. When I added acid fertilizer this year, my Columbine plants around the Rhododendron died. Could that be because the Columbine plants didn't like the acid soil? None of the other surrounding plants in this small bed were affected by the acid fertilizer. Would it help to put an underground barrier around the Rhododendron?

Most columbine are somewhat adaptive to a range of soils, although I suppose there could be some species that require a more alkaline soil...there are many. No, I don't think putting a barrier around the rhododendron would serve any purpose, and as it grows, the roots will grow to extend out as far as the widest branches - rhododendrons have a surface network of wide, shallow feeder roots you wouldn't want to restrict.

It's hard to answer your questions though with no gardening zone known for you, and you don't say what your approximate soil ph might be now. I don't know if you are in a harsh winter climate and trying to grow a rhododendron that isn't bud hardy for you there or just why it may not be flowering.

They are not heavy feeders and will often need no fertilizer in soils of average fertility. Additional nitrogen is rarely recommended unless indicated on a soil test - nitrogen can even be counterproductive to blooming, directing the plants energy into foliage growth, buds.

Drought in summer when buds are being set can cause failure to flower. Adequate water, moisture, but in soil that drains well and doesn't remain soggy is needed. Pruning at the wrong time of year, too much shade will result in no blooms. And as above, wrong plant for hardiness needed for your climate.