The volume of a potting container significantly increases the height of tomato seedlings during the first month of growth

Introduction & Background

I thought it might have occurred by chance.  Gazing at seemingly motionless tomato plants day after day I started to notice things I didn’t before.  I stumbled upon this peculiar pattern by accident after planting tomato seedlings in my basement.  It seemed as though the small plastic pots grew tomato seedlings faster than those in smaller seed trays, holding constant the type of potting mix, lighting, and watering.  This could have happened to anyone, but it happened to me and I like statistics, plus I had a yardstick handy. To understand how one could even stumble upon such a pattern, or why anyone would grow tomatoes in their basement, first some background.

Tomato seedlings used in the study. Note seed tray on the left and potting containers of the right.  This picture was taken after 28th days of growth.

Tomato seedlings used in the study. Note seed tray on the left and potting containers on the right. This picture was taken after 28th days of growth.

I had been gardening with my friend and I wanted to grow my own heirloom tomatoes.  Naturally, I read several books about growing them.  Since I didn’t have a backyard (well I had a 4 x 6 ft cement pad) for a greenhouse, I decided to grow them under eight 40-watt fluorescent plant/aquarium bulbs (48 inch) in my basement.  These bulbs are ideal on a small scale, because they emit visible light at wavelengths that are catered to those preferred by plants and do not get very hot.  Therefore the relatively low intensities of the bulbs can be made up for by the fact that they can be placed very close to the plants.

The fluorescent bulbs used for growing the tomatoes, along with the timers.

The fluorescent light bulbs used for growing the tomatoes seedlings indoors, along with the automatic timers.

Now when one becomes acquainted with heirloom tomatoes, one will most likely want to grow and taste as many different types as possible.  I mean have you ever seen, or more importantly tasted, a fresh Southern Night tomato? Or Striped German? The classic Brandywine tomato has got curves that will arouse, and a taste that will keep you coming back. Especially after picking it fresh from the vine.  Moreover, today you could potentially grow a wide variety of these unique plants by buying seeds from Seed Savers Exchange on the Internet.  Logically, Todd Wilson and I planned to grow as many varieties (this is going on over three years now with eight or nine) as possible.

The first year growing seedlings under lights was a wreck.  I planted seeds inside that should have been sowed outdoors.  I produced a swamp in my growing tray that propagated fungi (or something which smelled bad), which killed a few seedlings.  In another instance, I planted and then watched lettuce grow, bolt, and go to seed without eating it (I didn’t even keep the seed).  I still don’t know why.  Maybe I thought lettuce just grew then stayed tasty forever. I was very wrong.

The second year I refined my skills and developed a well thought out plan.  I would plant one heirloom tomato type and one variety of heirloom pepper.  The types planted; the classic Red Brandywine Tomato and the King of the North Red Pepper (both from Seed Savers Exchange).  I planted the tomato seed in each container (10.2 x 10.2 x 7.6 cm) and filled it with Vigoro Organic potting mix (0.10:0.05:0.05) I bought from Home Depot.

At the same time Todd Wilson was growing similar heirloom tomato seeds in a basement under fluorescent lights, except he used “seed trays” (or trays with small cylindrical pot holes compacted together) to start his plants.  This practice is a mainstay in farming operations in Pennsylvania, in which a large amount of plants have to be grown in a greenhouse during the early spring.  Under these circumstances, in may be hard to use a larger container, because of space or potting mix constraints, therefore seed trays are used so that there is a small volume of potting mix (and surface area) per seedling.

During the second year, Todd and I remarked that my tomatoes appeared larger on average than those he grew in seed trays (2.5 x 2.5 x 5 cm) under relatively similar conditions (although differences in heirloom varieties and lighting).  A few ideas flew around as to why one set grew faster than the other.  Eventually we settled on fertilizer differences (twice I used Miracle Grow while watering the seedlings) or the size of the potting container.

From there I began my search for understanding how to grow tomatoes faster and more efficiently.  With more data, I thought, this finding may be used to find the necessary level of potting mix needed for optimal tomato seedling growth, while minimizing excess potting mix use (e.g. volumes greater than X cm3 do not change the average height).  Think of the potential – tomatoes a week earlier!

Methods & Results

In April of 2013, I devised a strategy to find out what may have caused the pattern Todd and I observed the year before.  I would sow two heirloom tomatoes in a small plastic potting container (i.e. volume = 10.2 x 10.2 x 7.6 cm = 791 cm3) for every one I planted in the seed tray (i.e. volume = 2.5 x 2.5 x 5 cm = 31 cm3).  I employed six varieties of heirloom tomato and grew them under the same conditions.  In total, I planted 61 tomatoes split 40 in containers and 21 in seed trays.  I used Vigoro Organic potting mix and set the plants under an automatic timer that had the light bulbs on for 15 hours a day.  I then devised a system so that I could raise or lower the lights to about 2 to 4 inches above the plants to adjust their level as they grow.

The 10.2 x 10.2 x 7.6 cm container compared against the 2.5 x 2.5 x 5 cm seed tray used to grow the seedlings.

The 10.2 x 10.2 x 7.6 cm (or 791 cm^3) container compared against the 2.5 x 2.5 x 5 cm (or 31 cm^3) seed tray used to grow the seedlings.

I watered the plants as evenly as possible and twice added a tablespoon of Schultz Plant Food plus (20:30:20) (per gallon of water during watering) that I bought at Dollar General probably around 2002.  I placed a small fan with a timer on a table across the room, and set the timer to three 15-minute periods of oscillating wind over a 24-hour period.  I labeled the containers and trays carefully and took note of the day I sowed the seeds.  Also, I usually let tap water sit in an open container overnight, so that it would evaporate off the added chlorine.

After 26 days of growth (April 5 to May 1st) I measured each plant using a yardstick.  I placed the yardstick against the base of the plant and measured up to the highest point.  All measurements were logged in a notebook.

At a later time, I typed the tomato measurements into a Microsoft Excel spreadsheet, and converted the measurements centimeters.  I then broke up the measurements into two groups – containers (volume = 791 cm3) and seed trays (volume = 31 cm3) – which were then broken down further into heirloom variety.  These included; Mariglobe, Italian Paste, Mystery Brandywine (seed saved from feral Red Brandywine tomatoes in 2012), Red Brandywine, and Southern Night tomatoes.  Average heights were calculated per variety and per potting volume, along with error bars (Figure 1). This demonstrates that within 95% error, the heirloom tomatoes grown in containers with a volume of 791 cm3 were taller than those grown in seed trays with a volume of 31 cm3 (Figure 2).

Figure 1) Average heirloom tomato seedling height after 26 days broken down into large (i.e. 10.2 x 10.2 x 7.6 cm = 791 cm3) and small container (i.e. 2.5 x 2.5 x 5 cm = 31 cm3).  Error bars are shown to approximate the range of seedling measurements per heirloom type.  A total of 61 seedling were measured.

Figure 1) Average heirloom tomato seedling height after 26 days broken down into large (i.e. 10.2 x 10.2 x 7.6 cm = 791 cm3) and small container (i.e. 2.5 x 2.5 x 5 cm = 31 cm3). Error bars are shown to approximate the range of seedling measurements per heirloom type. A total of 61 seedlings were measured.

What if this occurred by chance? 

Could this have happened by chance?  Definitely.  We are certain that we are uncertain about everything.  However, can we determine whether or not that chance was so low that it most likely happened because something caused the pattern to emerge?  Yes.

For instance, to approximate results from individual experiments we often calculate the average.  This has disadvantages and, in most cases, does not reflect upon the reality of the individual data.  There is usually a distribution of values, in which some are probably quite different.  Therefore, to account for our uncertainty of the true average (if we measured an infinite amount of seedlings) I calculated the 95% confidence error bars.  Or rather, the two values that I am 95% certain the true average height falls between (Figure 1).  When the 95% error ranges of the two averages do not overlap, can we be sure that the plant height results are really different?  Probably.  And probably is not good enough.

Figure 2) Tomatoes seedlings grown in the larger potting container are significantly larger after 26 days of growth.

Figure 2) Tomatoes seedlings grown in the larger potting container are significantly larger after 26 days of growth.

To me, the height difference could have still occurred by accident.  Fortunately the t-test for two sample means exists, and by default assumes that there is no difference between the average plant heights.  After conducting the calculations and gathering the results (i.e. t = 4.45, df = 21, P = 0.0004), I can state with over 99% confidence, that the average plant height difference between the two container volumes is significant and has a very low probability of occurring by mere chance (Figure 2).

What caused the average heights to be different?

This is where it gets really interesting. What caused this to happen? I don’t know. I am confident that there is a significantly different tomato seedling height between those grown in potting containers versus seed trays after 26 days in a basement under eight 48-inch fluorescent bulbs that emit light for 15 hours per day.  Beyond that is all conjecture.

Currently, I think it is one or some combination of water/nutrient holding capacity of the potting mix, initial root growth dynamics, or how close the seedlings were to the light.

Of the three ideas, I don’t really think it is the water/nutrient issue.  I kept the soils moist during most, if not the all, of the 26 days. Plus, I used Schultz’s plant food.  This is assuming that there was some limiting nutrient, which is not known to me at this time.  Overall, it does not appear to be the problem to me.

The root-growth idea is a more favorable idea of mine at this point.  I say that because the sudden growth spurt does not become apparent until about 14 to 20 days of growth.  During first week of growth the seedlings are the same height.  However, it is during this time that the initial root growth protrudes into the surrounding potting mix.  Upon building a proper root infrastructure, the plant can then begin taking up nutrients to grow leaves and a stem.  I think that a certain amount or volume of root growth is needed to facilitate an optimal growth rate for the tomato seedling.  Restricting the volume of the early root growth is reflected in the poor growth during the first month.  On the bright side, though, after the seedling is “potted up” or put into the ground, it would make sense that normal growth rates would probably return.

Alternatively, a new idea is quickly gaining ground as a new favorite.  Could this have occurred because the containers were taller (by 1 inch) than the seed tray?  This situation is most precisely described by the light intensity equation, which states that light intensity is inversely related to this distance from the source squared (intensity = watts of light/[4 x π x distance2]).  So lets say we had an 80-watt source of light directly 4 inches above the potting mix surface in the container (i.e. 391 cm3).  Since the seed trays are 1 inch shorter, they would be 5 inches from the light.  Using the equation above, the intensity of the 80 watts of light would be reduced by 64%!  I conclude from this that the distance from the source of light is especially important during indoor growing, but how much this could account for differences observed in grow rates is not understood (by me, at least).  This, again, comes down to a limitation question. Was the growth rate limited by light that was further reduced by the longer distance traveled to the shorter seed tray containers?

On the other hand, in an outdoor greenhouse this should not make much of a difference.  This needs further investigation by people who have greenhouses or live in more favorable climates for growing tomatoes outdoors in the early spring.  Future research will figure out the influence of light intensity using fluorescent lights at different distances and with a range potting volumes.


This story and experiment has been to show you that using bigger potting container significantly increases the growth rate of tomatoes seedling during their first month of growth.  Faster growth rates probably reflect a tomato plant that is more efficiently transforming light, soil, and water into delectable fruits.  Gardeners who want to grow tomatoes faster take notice – potting containers are vital!


Posted on June 17, 2013, in Uncategorized and tagged , , , , . Bookmark the permalink. Leave a comment.

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