Potting Up in Container Gardening

yucatanApril 11, 2012

How often should container fruit/veg plants be potted up? I think I tend to over-do it and my concern is shocking the plant. Perhaps I should skip some of the in-between and focus on three transplants?

I have several pots of various sizes. In particular, my tomato plant went from jiffy seed tray, to 3-in diameter pot, to 6-in diameter pot, to 10-in diameter pot, to 14-in diameter pot, etc. throughout its growth cycle.

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DaMonkey007(10b - Miami)

If you use the mediums recommended on this forum, potting up is not necessary at all, outside of the jiffy to 3" pot step. The fact that the gritty or 5-1-1 are both free draining and well aerated will provide a perfect environment for plants at any stage of development. The reason that folks have to pot up there plants incrementaly in "slow" soils is to avoid the root problems that are created by an unfavorable root/soil mass ratio. To maintain hydration of a small plant in a large soil mass generally requires you to also maintain a significant portion of the soil mass in a saturated state, thereby inhibiting root function, promoting root problems, and limiting water/nutrient uptake in that region.

You don't fight this battle with Al's mixes. You can water freely without concern that the establishing fine root system will be drown in the lower reaches of the pot. Additionally, you don't have to force such a stressful life on your plants by constantly disturbing thier roots during devolopment.

As far as tomatos go, many folks around here, including myself, go from a 3"-4" nursury pot straight to a 15 gal container (or whatever) with great success.


    Bookmark   April 11, 2012 at 12:51PM
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This alleviates a lot of future work for me. Thanks a lot to you and Al.

    Bookmark   April 11, 2012 at 1:25PM
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tapla (mid-MI z5b-6a)

Soil choice determines what size pot or what soil volume is appropriate. In particular, the amount of perched water a soil holds governs whether a given soil mass is appropriate for a plant. The more water=retentive the soil is, the more critical the choice. As a soil approaches the point where it holds little or no perched water, there is no upper limit to the soil volume that's appropriate, from the perspective of root health/function.

You can put small seedlings in a large volume of well-made gritty or 5:1:1 mix with no problems, but the same can't be said of those soils that support considerable amounts of perched water. For those, if you are concerned about over-potting, you might want to take steps to reduce some of the effects.

I'll leave a link below to a thread that offers tips.

Here is something I wrote that discusses appropriate container size:

How large a container can or should be, depends on the relationship between the mass of the plant material you are working with and your choice of soil. We often concern ourselves with "over-potting" (using a container that is too large), but "over-potting" is a term that arises from a lack of a basic understanding about the relationship we will look at, which logically determines appropriate container size.

It's often parroted that you should only move up one container size when "potting-up". The reasoning is, that when potting up to a container more than one size larger, the soil will remain wet too long and cause root rot issues, but it is the size/mass of the plant material you are working with, and the physical properties of the soil you choose that determines both the upper & lower limits of appropriate container size - not a formulaic upward progression of container sizes. In many cases, after root pruning a plant, it may even be appropriate to step down a container size or two, but as you will see, that also depends on the physical properties of the soil you choose.

Plants grown in slow (slow-draining/water-retentive) soils need to be grown in containers with smaller soil volumes so that the plant can use water quickly, allowing air to return to the soil before root issues beyond impaired root function/metabolism become a limiting factor. We know that the anaerobic (airless) conditions that accompany soggy soils quickly kill fine roots and impair root function/metabolism. We also know smaller soil volumes and the root constriction that accompany them cause plants to both extend branches and gain o/a mass much more slowly - a bane if rapid growth is the goal - a boon if growth restriction and a compact plant are what you have your sights set on.

Conversely, rampant growth can be had by growing in very large containers and in very fast soils where frequent watering and fertilizing is required - so it's not that plants rebel at being potted into very large containers per se, but rather, they rebel at being potted into very large containers with a soil that is too slow and water-retentive. This is a key point.

We know that there is an inverse relationship between soil particle size and the height of the perched water table (PWT) in containers. As particle size increases, the height of the PWT decreases, until at about a particle size of just under 1/8 inch, soils will no longer hold perched water. If there is no perched water, the soil is ALWAYS well aerated, even when the soil is at container capacity (fully saturated).

So, if you aim for a soil (like the gritty mix) composed primarily of particles larger than 1/16", there is no upper limit to container size, other than what you can practically manage. The lower size limit will be determined by the soil volume's ability to allow room for roots to 'run' and to furnish water enough to sustain the plant between irrigations. Bearing heavily on this ability is the ratio of fine roots to coarse roots. It takes a minimum amount of fine rootage to support the canopy under high water demand. If the container is full of large roots, there may not be room for a sufficient volume of the fine roots that do all the water/nutrient delivery work and the coarse roots, too. You can grow a very large plant in a very small container if the roots have been well managed and the lion's share of the rootage is fine. You can also grow very small plants, even seedlings, in very large containers if the soil is fast (free-draining and well-aerated) enough that the soil holds no, or very little perched water.

I have just offered clear illustration that the oft repeated advice to 'only pot up one size at a time', only applies when using heavy, water-retentive soils. Those using well-aerated soils are not bound by the same restrictions.


Here is a link that might be useful: Dealing with water-retentive soils.

    Bookmark   April 11, 2012 at 2:35PM
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Makes sense. I've used the 5-1-1 in the past and potting up beyond the seedling stage was always a nightmare. As stated, I did this quite often and tons of very loose potting mix was left to clean up afterward. Glad to know it is unnecessary in this particular case.

    Bookmark   April 11, 2012 at 2:44PM
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