Plant containers – does size really matter?

A few days ago I got a question from Cynthia about “potting up.”  For those of you for whom this is an unknown phrase (and no, it’s not a euphemism for a certain herbal activity), it refers to the practice of moving plants into ever larger containers.  She was wondering if there was any “real science” behind the practice – in other words, why not just start out with a larger container?

Hah! I needed no further encouragement and spent several days collecting and reading decades’ worth of research. And there is a LOT of research on this topic. As you might guess, it’s geared towards production nurseries and greenhouses.  But the good part is that it’s been done on just about any kind of plant material you could want.  Vegetables.  Annuals.  Perennials. Grasses.  Shrubs.  Native plants.  Ornamental, fruit and forestry trees.  Seeds, seedlings, cuttings, big plants, little plants.  Ahhhhh…data!

Almost without exception, you get better growth on plants grown in larger containers, whether you’re measuring height, number of leaves, leaf area, stem diameter, shoot and root dry and fresh weights, whole plant dry and fresh weight,…you get the idea.  This isn’t surprising, because with a larger root zone you can support more roots, which in turn support more above-ground growth.

The only parameters which tended to diverge for some species were flower and fruit production.  Restricted roots can stimulate sexual reproduction in plants, possibly because poor growing conditions spur the plant to reproduce before it dies.  Other drawbacks include increased probability of circling root systems, and higher ambient soil temperature, compared to plants in larger containers.

Smaller containers might be considered desirable when one is trying to limit above-ground growth – the “bonsai” effect.   And they require less water than larger containers – which brings us to the bottom line, as far as production nurseries are concerned.

Larger containers take more space.  And water.  In at least one study, water costs were shown to be “prohibitive for larger container sizes.”  Furthermore, smaller containers are preferred by production nurseries to “optimize production space.”  Another economics-based study found that “the smaller of these was the more economical.”

But most of you probably aren’t interested in the economics of plant production – you want to know what’s best for your own container plants, whether they are houseplants or pots of herbs or punches of annual color on your patio.  The science is clear:  it’s best to pot up plants in small containers quickly into their final destination, rather than making several (pointless) intermediate transplants.

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

26 thoughts on “Plant containers – does size really matter?”

  1. Are you saying I should pot my tomato seedlings directly into the 5 gal. pot I’ll keep them in all summer? I’ve always jumped from cell packs, to 4″, to gal. to final pot. That would save a lot of time, but I worry about over watering small plants in big containers.

  2. Nancy, apparently yes. What you’ll probably see is little top growth initially as roots go crazy, but then the top should catch up. I agree you want to watch out for soggy soil, so be sure you use a well-drained media. Add an organic mulch as the plants get better established to reduce evaporation.

  3. LInda – thanks so much for the great article! I was just thinking about this issue as I was planting my peppers seeds this weekend. I’ve always planted vegetable seeds into 3″ or 4″ containers, and the plants stay in them until they go into the garden. My common sense told me it was silly to start with 1″ cells, then replant the seedlings 1-2 more times before planting outside, but gardening books and my mother always told me differently. But now I have scienc
    19e0
    e backing me up – I’ll make sure to pass this on to my mom!

  4. I think one exception might be tomatoes. Each time I repot (from seed tray to 4 pack, to 3in, to gallon if necessary) I plant very deeply, up to 80% in soil. I get extremely sturdy, stocky stems doing this. If you plant the seedling in the five gallon pot you do not get the advantage of planting deeply several times. People are always amazed at the size of my plants and the number of tomatoes I get off of each one. Would be interested in the data on tomato plants.

  5. Too late for me, a newbie who started my pepper and eggplant seeds in cells. Now I have to put them up because we have nearly two months to go before planting in the garden. God to know I can skip that step next time. (By the way I looked at your companion planting page yesterday and would love to see more about that here.)

  6. Liz, my collection or articles does include tomato research. Of those that I could actually read online, they both found that larger containers were better for earlier flowering and better production. (The other articles were from the 1980s and are only available hard copy, so I haven’t seen those).
    Val, I’ll try to remember to do a companion planting piece sometime in the future. For those who haven’t seen it, it’s at http://www.puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Companion%20plants.pdf.

  7. Linda, the gradual potting-up argument put forth by some of the “experts”: a small plant in a large pot is susceptible to the effects of a lot of free-standing water, notably root rot. You make the point in these comments to use easy draining soil. Is there any research regarding the deleterious effects of “small plant-big pots-free water”?

  8. Fred, standing water is going to kill almost any transplant regardless of size. (None of the articles I looked at reported any problem like this.) Production nurseries, of course, have no issues with drainage because they use soilless media or amended soils in the pots to accelerate drainage. And – this is important for people to note – they don’t put gravel or other “drainage material” at the bottom. There’s no better way to create a perched water table – and flood the root zone – than by adding “drainage” material to the bottom of a container.
    My advice to gardeners is to use clean soilless media and nothing else in their pots. They won’t have drainage issues, they won’t have disease problems (reused media and garden soil can carry pathogens), and roots will establish quickly.

  9. One practical consideration, even for non-production home seed starters, is space under your lighting system. Using smaller containers allows me to start a greater number of plants indoors. By the time they need to be potted up, I can take advantage of warmer weather and longer daylight hours and my unheated cold frame space.

  10. Linda, you’re the scientist, but I think the relationship between precocious fruiting and restricted root growth may be more complicated or not the same thing as the plant “sensing” impending doom.

    The roots supply water, N, etc. the leaves the carbs. Restrict the roots and more carbs stay above ground changing the metabolism to that of a mature plant.

    Hastening a plants maturity needn’t be described as tricking a plant into thinking it’s dying. You could just as well say it’s tricking a plant into thinking it’s an adult but actually you are turning it into an adult of small stature.

    I bet you can describe the whole thing more elegantly, but is my premise correct?

  11. Alan, from an evolutionary biology perspective, the ability to reproduce is the only way to guarantee your genes are passed on. When environmental conditions aren’t optimal, you’ll often see precocious maturity in young plants or excessive fruit production in mature woody plants. It’s this last observation with mature plants that led to the “reproduce before you die” theory.
    I think you and I are saying the same thing. You’ve explained the “how” (the physiology behind the phenomenon) and I’ve described the “why.”
    Not sure if this is any more elegant, but I hope it helps!

  12. Linda,
    I’m not a professional gardener but a passionate gardener. Your post about the right size of plant containers was very informative to me and I learned a lot that hopefully will help my gardening experience in the future. I can’t remember anymore how many times I have replanted plants in bigger containers, simple because they rooted out or the plants just wouldn’t do good in containers too small for them.

  13. As with many horticultural issues, I think the real answer is, “It depends.”

    By chance, I posted a report on our cut flower research here at Cornell yesterday morning and then read your post last night. One of the 2010 experiments was to determine the effect of holding transplants longer and/or in smalle
    r cells.

    In the case of Zinnia ‘Uproar Rose’ and lisianthus ‘Echo Champagne’, no
    yield decline if transplanting delayed by 3 weeks, even if sown in a 200‐cell tray. But with godetia ‘Flamenco Salmon’ and larkspur ‘Sublime
    Dark Blue’, yields were reduced.

    Find links to the full report here: http://blogs.cornell.edu/hort/2011/03/24/cut-flower-report/

  14. Thanks Linda. It’s funny, but because I spend all my professional time trying to manipulate fruit trees to bear copiously and with high quality, I don’t think of the manipulation as creating stress. Some of the things that hasten maturity, such as spreading branches and avoiding heading cuts are not stressing plants. Might be the opposite, but getting the same result.

    I can certainly see how restricting the roots would be considered a source of plant stress, however. You could even say that dwarfing rootstocks stress the scion but the line starts to become less clear.

  15. Wow…. Deja Vu…. well, coincidence….

    Less tahn a hour ago I was viewing a segment on Good Morning America on a clothing maker featuring padded bikini tops for 6 to 10 year girls. This lead to a broader discussion of marketers targeting ever younger audiences with “sexually” oriented products like suggestive clothing, shoes etc. Even one off the wall mega-mart we all love to hate has a line of make-up that targets girls as young as 6 with products to make them more “appealing”. Then Alan states “Hastening a plants maturity needn’t be described as tricking a plant into thinking it’s dying. You could just as well say it’s tricking a plant into thinking it’s an adult but actually you are turning it into an adult of small stature.”

    Then Linda comes back with “When environmental conditions aren’t optimal, you’ll often see precocious maturity in young plants or excessive fruit production in mature woody plants.”

    My mind is now full of thoughts of my young mellons aupported by padded bikini tops, my tomatoes wearing support hose and excessive make-up on my Halloween Pumpkins. I’m sorry folks, its just the weird confluence of subject matter pummeling my brain that set this off. Really I’m not a plant pervert…. I really do want my melons mature before I eat them.

  16. Old thread, but here’s my .02.

    There’s a guy on Youtube – mhpgardener – who grows tomatoes in hoop houses. He puts two indeterminate plants in a single 5 gallon grow bag, trains them to one stem each, and gets fantastic prodution. I mean 4-5 fruit every 10 inches or so, going up to the top of the hoop house. This amazed me, but also made me think. Plants need roots large enough to draw the necessary water/nutrition. If you’re providing both in sufficient quantities, then how extensive does the root mass need to be? Bobby waters from the bottom (his grow bags sit in plastic basins) and adds fert. during the season. This is a good ole boy in Virginia experimenting on his own, and his production is amazing.

    All things held equal, I have no doubt that bigger is better. But gardeners are able to manipulate their plants so that all things are not equal. In that case, smaller may be just fine.

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