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.

Mouse Tombstones

Doing this…

I must identify each and every Wave petunia!

leads to this…

and this…

Unfortunately, the second and third photo are from a college teaching and display garden. There are a lot of inexpensive yet more attractive ways to relate plant identification to your visitors.

We all do this, of course, to some extent in our home gardens. Walk around with weird plant in hand, dig hole, stick it in, say to self "I’ll come back and get that label to add to my (pick one) Excel garden database/bag of random tags/photographic memory."

Typically, most of mine end up on top of the washing machine from cleaning out pants pockets. Not exactly a good record-keeping system.

Tags as tombstones: during spring mulching, I’ll typically find a few half-buried, printed with, oh, Maybehardii maybenotus – Plant Delights Nursery $25.  Said plant is nowhere to be seen. Dang.

Heck, it’s hellebore!

You were too smart for me this week.  Though my husband was convinced that Friday’s photo was not enough to help identify the plant, JRR, Foy, and an unnamed commenter all recognized hellebore:

Always a welcome sight in the spring.  And don’t be fooled by those old wives’ tales that hellebores don’t like wood chip mulches. These beauties have been in wood chip mulches all their lives, and not only do they do great, their seedlings do as well.

Bordeaux Mix

One of my favorite stories about pesticides is the story of Bordeaux mix.  It’s a story of France in the 1800s (so it must be pretty romantic, right?) and how they were suffering from a shortage of grapes.  Don’t feel sorry for them — it was really their own doing.  Over the course of the 19th century grape vines were brought from the United States to test their merits against European grapes.  It was quickly discovered that, for the most part, American grapes were not the equal of European grapes for winemaking.  Unfortunately for the French, however, along with the grapes came a disease: downy mildew.  This mildew absolutely ravaged grape vines across Europe, and particularly France from the time that it was introduced, around 1878.

Meanwhile there was another problem for grapes growing in France.  People.  People like to eat grapes beside the side of the road and so, throughout France’s grape growing regions, grapes on the sides of the road were typically bare.  Unlike downy mildew, however, grape growers had a pretty good idea what to do about people.  They sprayed nasty stuff on the grapes.  This nasty stuff took many forms, but the one which was most effective was a mixture of copper sulfate (basically you dissolve copper in sulfuric acid) mixed with lime.  Brushed on a plant’s foliage, it was darn ugly.

Then came 1882; a terrible year for downy mildew.  Grape vines were losing their leaves all over Europe, except for those vines beside the sides of the road.  There the grape vines were doing just fine.  The reason was the copper in the lime/copper sulfate mixture which was eventually dubbed Bordeaux mixture because of where it was first used.  Bordeaux mixture is still available today, and is one of the most important tools in the organic grower’s pesticide arsenal.  Unfortunately it’s nasty stuff – it builds up in the soil and it’s toxic to earthworms and a wide variety of different plants and aquatic organisms.  Using this stuff once in a while – such as once a year – isn’t terrible, but regular use is a good way to ruin your plot of land.

One final thought – Those American vines which originally brought in mildew?  They eventually became very important to French wines because of another introduced pest, phylloxera.  They were used as rootstocks because they were resistant to this pest — unlike European grapes.

A salt bath for your tomatoes?

This morning I got an email from one of my gardening colleagues, wondering about the wisdom of watering tomato plants with salt water.  He had a link to a UC Davis website which tacitly endorses spraying tomato plants with 10% salt water “to increase their nutritional value and taste.” Unreferenced “worldwide studies” are mentioned, along with the “major potential benefit of providing irrigation for crops in areas with freshwater restrictions.”

Before we deal with the impracticalities and out-and-out harm of using salt water for irrigation, let’s look at why this practice would work on tomatoes.  By training I’m a plant stress physiologist (and I’m well versed in the primary literature on this topic).  Watering tomato plants with a salt solution imposes a drought stress on the entire plant, as less water is taken up under these conditions.  So leaves and fruits are smaller and they may produce stress-induced biochemical compounds in response.  The upshot is that you have smaller tomatoes with a higher concentration of various solutes, some of which might be tastier or more beneficial to humans.

Guess what?  You can do the same thing by decreasing irrigation during fruit set!  Less water means smaller fruit and increased concentrations of sugars and other plant compounds, and voila!  So you can skip that step of adding salt water and just cut back on irrigation to induce a mild drought stress.

So…why in the world would you dump salt water on your garden soil?  The article blithely dismisses this:  “Many are still concerned about salt causing soil degradation and rendering some seawater-treated tomatoes inedible, but scientists cite that plants thrive in balanced soil containing both macro- and micronutrients.”  Sorry, but sodium is NOT a micronutrient for most plants and does NOT contribute to a “balanced soil” in one’s vegetable garden.

An ironic twist to this whole article is that most of the research that’s been done is relevant to arid parts of the world (the Middle East, primarily) where saline soil conditions and limited water are common.  I can’t imagine what they would think about people who would deliberately contaminate good soil by adding salt water to it.

Jeff Gillman a.k.a. Dr. Unbiased!

“Everyone’s taking stands, and unfortunately, some of those taking the strongest stands have the least information.”
Dr. Jeff Gillman, on “How the government got in your backyard”, co-authored with Eric Heberlig

Fine Gardening did a fine job in a recent interview that was linked to their e-mail update.  FG Editor Steve Aitken brings some humor as he quizzes Dr. Jeff. about his new book.  Check it out here.

Steve opens with “Is the government really in my backyard? And if so, can I get them to pull some weeds?

Hee!!!  The interview awesomeness continues as herbicides, nudity, tofu dogs, and  poo-pooing are all discussed. I just may renew my subscription, if Steve can promise more of the same. I gave up on Garden Design years ago – since I don’t have a gravel garden with infinity pool overlooking a canyon in California, nor am I interested in $5000 lawn chairs. Reading it just made me feel dowdy.

“I can practically guarantee that you’ll find something in this book that
you don’t like.” 
Way to sell a book, Dr. Jeff!

Despite waiting pensively by the mailbox for my Complimentary Copy which has yet to arrive, I can safely say there’s already a kerfuffle brewing over the book. As Jeff noted in his post last week, folks have already weighed in DISAGREEING with his position. Wait! He has no position! That’s the whole bloody point. It seems an alarming number of people aren’t sure what to do with the 1,350 grams of gray stuff between their ears.

Relative to last weeks “trivia” post: I’m apparently the tallest Garden Professor, at 72 + 5/8 inches.