Soil depth for onions in containers?

anney(Georgia 8)February 26, 2009

I posted this on the Vegetable forum and got one answer but thought people in this forum might have more experience in container growing than those of us who grub around in garden soil.

Here's the post:

Since I have heavy red clay, I've been planning to grow onions to full maturity in ProMix in some large bins so they have nice loose soil to expand in. I am however clueless about how deep the growing medium ought to be.

I haven't been able to find anything about a recommended depth of soil for onions other than at the link below. And it doesn't actually recommend a soil depth. It just notes that in the varieties of onions they observed, the roots reach as deep as 39 inches! Whether it's relevant or not, it appears that onion roots for the most part spread laterally rather than vertically, so I'm wondering how important vertical space is for them. No way can I grow onions in ProMix that's 39 inches deep since my bins aren't that deep!

If you've grown onions to maturity in containers, what depth soil have you provided for them -- and did they grow well?


Someone commented that most root crops require 6-8 inches of soil for their root systems. I thought that might be fine for a raised bed but didn't sound like enough growing medium for onions in a container. If it is, that's great, but I wanted to check it out.

Here is a link that might be useful: Onion Roots

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I haven't grown onions in containers, but I can't imagine 6-8" wouldn't be enough. Some "root" crops will want more depth such as carrots and potatos, but other things like radish, onion and garlic should do fine with 6-8".

I know of people who grow onions in Earth boxes and those have a 6" potting mix depth. Anything where the roots can spread laterally doesn't need a lot of depth, just sufficient volume of soil to hold water and nutrients until you next add them.

    Bookmark   February 26, 2009 at 10:00AM
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anney(Georgia 8)


I had to do some digging for the information but did find the site below, UC-Davis, and have decided to go with a 12 inch depth of soil for onions in my container.

The recommended depth for them is 9-12 inches (partway down the page). There is an implication that the deeper than expected soil levels are recommended since they're less likely to quickly dry out. IOW, I don't see anything stating that the roots need this much soil, but I think I'll go with it anyway.

Here is a link that might be useful: Grow Vegetables in Containers

    Bookmark   February 26, 2009 at 2:47PM
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Should work just fine, Anney.

    Bookmark   February 26, 2009 at 2:50PM
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franktank232(z5 WI)

Here was mine. Been using the same container for years.

I think this are just plain Spanish type onions. The biggest i've grown have been the size of a softball (in this container). EASY to grow (i grow them from seed), transplant and give them full sun...water once in awhile...

    Bookmark   February 26, 2009 at 8:27PM
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anney(Georgia 8)


Very nice! Soil about 6 inches deep?

    Bookmark   February 26, 2009 at 9:21PM
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bumble_doodle(Z5 CT)

Okay, this is probably a really stupid question. Last summer I grew onions for the first time from sets. The containers were about 6 inches deep and I buried the set so just a tiny bit of the top was sticking out of the mix. The onions grew but they were on the small side. Now having seen franktank232's GORGEOUS photo I noticed all the onions are on TOP of the soil. What gives? Am I supposed to just "set" the sets on top of the soil and let the roots grow downward?

Please help this onion newbie!

    Bookmark   February 27, 2009 at 9:49AM
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anney(Georgia 8)


It's my understanding that onion SETS don't do as well as onion transplants, often producing very small onions, sometimes large ones, depending on several factors.

All the "growing onions" sites say to plant the sets or transplants in the soil, though not very deep. [Not on top of the soil.] 1" deep for transplants is recommended, and maybe someone else who plants sets can give you a depth for them.

Some people remove the soil around the tops of onions after a while to keep an eye on how large they grow, so I don't think FT planted his on top of the soil, but he'll have to answer you himself!

    Bookmark   February 27, 2009 at 10:31AM
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franktank232(z5 WI)

About 6 inches...

I don't bury them at all, just put them in at the level they were growing in before transplanting. The key i guess is if you are starting from seed, do it now! The earlier, the bigger the onion... Onion roots don't seem to get that i think they are perfect for container growing. Get about 20 plants and 4 or 5 of these small containers...throw in some compost.

June 9


I'm growing Alisa Craig this year, i guess they are world record sized onions or something like that.

    Bookmark   February 27, 2009 at 12:00PM
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dan_staley(5b/SS 2b AHS 6-7)

Even after seeing Frank's piccies, I still lean toward what justaguy wrote above. Everyone has their own system, but until you get it down you should go with what the pros say until you are ready (sez the UC Davis alum). Even though onions are pretty resilient, I have some extra onions from seed and I'm going to throw them in some extra 7-gal landscaping pots I dumpster dived freecycled and see what happens...that said, I'd be interested to read what anney ends up with and how successful it is.


    Bookmark   February 27, 2009 at 1:59PM
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bumble_doodle(Z5 CT)

Thanks for the tips - I didn't know that about sets vs. seeds. I was planning on using sets again but I may have to reconsider.

It's hard to believe it's already time to start thinking about gardening. Last night's Nor'easter dumped 12" of snow on the ground and another 3"-4" is on the way. Grrrrrrr.

    Bookmark   March 2, 2009 at 12:45PM
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