Container Growing

whitecap2March 28, 2012

I've been growing azaleas in 20" containers for several years now, with mixed results. They bloomed very well this year, but the new growth emerging on the 3 year old plants is pale green, somewhat yellowish, which is usually a sign of drainage issues. They were repotted just last spring. The growth on the plants placed in these containers two years ago looks much healthier. (I start out with 6" pots.) I've been using potting mixes marketed as being especially prepared for growing azaleas. It has a nice, light texture to begin with, but when I have to repot, I invariably find that it has become very compacted, almost as though it had a lot of clay in it. I've experimented with blending sand into the potting mix, and drilling 3/4" holes into the sides of the containers. (The containers being concealed by planter boxes.) This doesn't seem to have helped much, although the growth rate, for the first couple of years, is usually quite satisfactory. I've been reading here that organic materials usually become compacted over time. This set me to wondering if one couldn't prepare a mix containing less organic material, with a view to obtaining better drainage, over time. I would suppose one could supply the necessary nutrients by other means. But what would be the composition of such a mix?

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morz8(Washington Coast Z8b)

Not 'suppose you could supply the nutrients by other means' - unlike azaleas in the ground, with containerized plants its essential that you provide the fertilizer needed, they are dependent on you for fertilizer and water. The better potting mediums for containerizing azaleas can be as much as 3/4s nutritionally inert material...you've got to fertilize. When - will depend on where you live and how harsh your winters are. A product you can mix with water and apply as a soil drench is usually safest, one for acid loving plants like Hollytone.

Accent is on porosity but with good water retention...Peat or ground bark are often the bulk of the mix. Sometimes builders grit to increase drainage. A good commercial mix should be providing that, are you sure its the mix itself that is packing down or are the fine roots of the azaleas themselves filling up the container after that many months, making for a tight rootball. Adding ground bark or bark fines to a commercial mix if you aren't happy with what you are purchasing should improve it (although that could be a personal choice, I prefer the ground bark over peat myself)

You don't say which zone or state you are in...containerized azaleas in areas where hot humid weather will coincide with summer rainfall can be especially problematic, and bark chips rather than bark fines may need to be incorporated into the potting medium to increase porosity.

With all container grown plants you will likely encounter a gradual buildup of harmful salts in the planting mix (which could also cause yellowing). To offset that, avoid shallow watering, water deeply enough some is running from the drainage holes each time you water. And periodically, flush the planting mix by filling pot the the brim several times over until water is freely running from the drainage holes. If your water is alkaline or has high mineral content, you may need to do that every month - every three or four months if your water quality is good.

    Bookmark   March 28, 2012 at 8:19PM
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whitecap2

I'm in 8b. I've lost several plants to root rot, and I'm convinced inadequate drainage is the problem. Peat moss and pine bark become, over time, very compact, and just don't drain well. Root binding doesn't seem to be a problem because, when I repot, I usually find that the roots have gone down no more than 8" or so. The thought I had in mind was substituting fertilizer for the nutrients the plants would otherwise obtain from organic materials.

    Bookmark   March 28, 2012 at 9:58PM
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mainegrower(Z5b ME)

morz8 has given very good advice about container mixes to which I'd only add this: the standard sort of bark used in mixes for ericaceous plants was for many years, bark from conifers. This was slow to decay and produced an acid reaction as it broke down. More and more I'm seeing so-called bark mulches which are ground up and dyed "natural forest products", aka wood. A lot of this is hardwood from discarded pallets, construction debris and other material which not suitable for azaleas or rhododendrons. Follow morz8's advice and use the mini bark nuggets if you're going to modify commercial mixes. You may, however, have to resign yourself to yearly or every other year repotting.

    Bookmark   March 29, 2012 at 5:53AM
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whitecap2

The repotting issue is what brought me here. I get good growth from the mixes I'm using. I'm just trying to get away from having to repot every other year. It has frequently been noted on this forum that organic materials, including pine bark, eventually become compacted and stop draining well. That is the issue. I understand that, if I substitute inorganic for organic materials in the "mix," I will be reducing available nutrients, and perhaps necessary acidity. I don't offhand see why this couldn't be remedied by the addition of sulfur, nitrogen, whatever. I've spent a little time on the Container Forum, where there is much enthusiasm for "gritty mixes" containing high proportions of decomposed granite and whatnot. I'm wondering if something like this might extend the interval one can go without repotting. I understand that one would get better results using a mix high in organic materials, but repotting is just becoming too much of a hassle. The containers are so heavy that I risk lower back issues by trying to lift them from the planter boxes. That leaves me the question of what I might add to the organic potting soils, which is why I'm here.

    Bookmark   March 29, 2012 at 6:50AM
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whitecap2

Hmmm. . .Perhaps a mix of Turface, perlite and "used" azalea soil? We will see.

    Bookmark   March 29, 2012 at 9:03AM
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reg_pnw7(WA 7, sunset 4)

Organic materials break down over time, that is their nature. And that leads to compaction. You might be able to reduce the overall compaction by adding finer grades of sand to the potting mix. Overall morz8's advice is excellent. You will have to fertilize, regardless of higher or lower organic content in the potting mix. I like little bits of pumice or perlite in my peat/bark mixes myself. No need to 'offset' loss of nutrients through higher mineral content since the peat and bark aren't supplying nutrients anyway.

Zone 8b - which one?? Zone 8 stretches from Seattle to South Carolina and I can guarantee you, growing conditions vary tremendously from one end to the other.

    Bookmark   April 1, 2012 at 1:04PM
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whitecap2

San Antonio. I have no problem getting good bloom and growth from the plants for a couple of years, before I run into drainage issues. I see some container rose enthusiasts boast of having to repot only every 4 or 5 years, and that's what I'm aiming for. I haven't questioned the necessity of nutrients. What I was trying to say is that, if I use more inorganic materials in the mix, I would have to increase the supply of nutrients. I've now decided that the use of stuff like Turface and decomposed granite would require more frequent watering than I have time for. (I've been getting by with no more than once a week in the summer.) So, back to the drawing board.

    Bookmark   April 1, 2012 at 6:33PM
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whitecap

I've been doing a little experimentation here, and think I may have hit on a way to avoid frequent repotting, and even reverse root rot. I posted a note on the Texas Gardening forum.

    Bookmark   March 17, 2014 at 9:24AM
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