Why pot on into a small pot?

FireFire LondonPosts: 5,402
I'm wondering about the science of potting on from a small pot to a slightly larger pot. What happens to plants if they are potted from a small pot to pot three sizes up instead? They say that for all plants - like large established shrubs -  one should go just one pot size up and not jump sizes as it will harm the plant. Surely if you can happily plant a shrub or a plug plant straight into the ground to thrive, then putting a plant from a small pot to a big pot should be fine. "Potting on" must have surely some good reasons behind it. Answers on a postcard please.


  • AlchemistAlchemist OxfordPosts: 193
    edited March 2018
    I’ve always questioned the science behind big pot harming plants. The only thing I have come across with a search is that -root restriction can reduce plant size without affecting the overall ‘produce’ in some plants. So I assume the argument could be, if you plants are doing well you are not gaining much by shoving it in a huge pot.  Rather you may alter the flower/fruit to leaf ratio and may end up with a bigger plant with similar produce. Source for this here: https://www.ncbi.nlm.nih.gov/m/pubmed/27345202/?i=1&from=pot%20size,%20root%20restriction
  • PosyPosy Posts: 1,618
    It is a bit of a mystery. The accepted wisdom is that the small plant will be overwhelmed in a big pot, but they seem to cope in open ground when they self seed. As for shrubs, once they are established, I don't bother with one size at a time but I don't go from 9cm to 30cm, I take it gradually and increase the ratio of grown up compost as I pot on.
  • LeadFarmerLeadFarmer Posts: 850
    If all the soil in the big pot gets soaked then could the soil get cold and too wet for the roots of the small plant?

    Maybe the nutrients in the soil could wash away before the roots get to grow into them?

    You use more soil/compost in one go?
  • DovefromaboveDovefromabove Central Norfolk UKPosts: 55,613
    The open ground is part of a bigger whole, teeming with micro organisms bacteria and invertebrates that all interact to create a healthy aerated environment ... an isolated pot of compost will not have all this and can become stagnant and sour quite quickly and new plant roots will not grow out into this unwelcoming 'sludge'. 

    Overpotting is not a short cut - it's a waste of compost and stunts growth. 
    “I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh

  • AlchemistAlchemist OxfordPosts: 193
    Could using a soil based medium with a grit/good helping of garden soil as opposed to just compost be a way of overcoming this?
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  • Busy-LizzieBusy-Lizzie Posts: 13,735
    When I plant out my bedding, such as pansies and bellis daisies into the final big pots they do well. Last autumn I had too many so I potted up the others for later use if necessary into small pots. The ones in the big pots are doing really well but not the others. Maybe I should have done a medium stage, but they haven't grown much at all.
    Dordogne and Norfolk
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  • FireFire LondonPosts: 5,402
    All interesting thoughts. Thanks
  • BobTheGardenerBobTheGardener Leicestershire, UKPosts: 7,980
    Why not do the science yourself, which would be rather easy if you have the space?  I'm sure the results will show that this is something many of us have discovered by our own experience.  When I first started growing peppers for instance, I initially potted the seedlings straight into their 8" pots but (as usual!) I had too many seedlings and not enough pots so the 'spare' seedlings went into 3" pots as those were all I had.  The latter continued growing but the ones in 8" pots stayed the same size for many weeks.  I think Dove is 'on the money' regarding the reasons plus a larger mass of compost will take longer to warm up which will restrict root growth.
    A trowel in the hand is worth a thousand lost under a bush.
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