The reason you pot-up is to bury the stems up to the leaves. The buried stems will grow roots and this results in a very strong root system.

I am starting seeds in a large pot, but what I will do is dig the seedling up and then plant it deeper a few times.

yumtomatoes:

What about starting seeds in a large pot as you are doing, but only filling the pot 2/3 of the way up at first. Then, as the seedling grows, add more soil to the pot to bury the stems. Seems like this would accomplish the same thing (eg strong root system) without stressing the seedling by digging and replanting.

I've read a few articles about starting tomatoes from seed, and all of them recommend starting in small containers and then moving the seedling to a larger container once or even twice.

Frequent question here always based on "easier". Many of the posts even include pics of the difference in root development. It may be easier but the point is to do what is best for the plant, not easier for the planter.

So the primary reason all the articles recommend as they do is because it is the act of transplanting - actually removing the plant from one small container to another larger one - is needed to trigger the fibrous/feeder root development in the plant. Don't do it and you end up with a tap root plant with minimal feeder root development.

Secondary benefits are stronger/larger root system, top growth is slowed allowing root development to catch up, shorter stem inter-node length assuming adequate light available, etc.

Do your seedlings the favor of transplanting them at least once. They will reward you for the effort.

Dave

digdirt:

Thanks for your excellent explanation as to why transplanting is beneficial. Sounds like it is the opposite of what I expected which is that it would stress the seedling.

I will follow your (and others) advice and transplant.

Well, it does stress the seedling, thats basically the point. When its stressed, the plant basically goes "that was unpleasant, I need to make sure I'm more prepared if that happens to me again!" and grows stronger as a result.

Much like with people, the tomato will do the bare minimum it needs to to get by. Problem there is, again much like with people, the bare minimum to survive during the good times is going to mean you can't cut it when times get more stressful.

I believe most of the discussion on transplanting causing a more fibrous root system is based on a study conducted in 1927. This study did show a more vigorous root system from transplanting, but if I recall correctly the transplanted plants did not yield more tomatoes. Also I believe it said that the fibrous roots also develop when the tap root hits a barrier, like the container bottom, which happens very quickly in ordinary pots (3-4"). I think the study used one gallon pots. Perhaps someone can link to this study.

I start all my tomato seeds in 3.5" pots and leave them. About 25% of my seedlings do get transplanted to fill empty center spots in the containers. In over 30 years of doing this I have never noticed any difference between those that were transplanted and those that were not. When I take my plants to the garden they are all the same size, the root balls look the same, and they all perform equally in the garden. Hence IMHO transplanting doesn't make much if any difference.

TomNJ

I found the 1927 study:

http://www.sustainablefarmer.com/pdflibrary/library-rootdevelopment.pdf

The section on tomatoes starts on page 180. The first several pages clearly shows that disturbing the tap root causes a more fibrous root system, however, transplanting also checks plant development. What really matters is the health and yield of the plants when in the field. In this regard the report states on page 192:

"Usually the seedlings are transplanted when they are about 2 inches high and more space given each. In this process the roots are more or less severely pruned and the new root system is much more fibrous. Frequently a second transplanting is made in which each plant is given still more room or transplanted in a suitable receptacle such as a flowerpot, paper band, tin can, etc.

Since the root system is disturbed and the development of the plant more or less checked at each transplanting, it might be concluded that plants grown from seed sown directly in pots or other containers would grow more vigorously and give a higher yield than those once or twice transplanted. In fact this has been shown by numerous investigations to be the case. That transplanting in itself does not promote an early crop nor an increased yield has been also clearly demonstrated.

In an experiment in Wisconsin three crops of tomatoes were grown . . . In each case seeds were planted singly in 6-inch pots in the greenhouse; when the plants were about 2 inches in height, two-thirds of the whole number were dug up and reset in the same pots; later, one-half of these were again transplanted in a similar manner. As soon as weather permitted, 10 plants of each lot were knocked from the pots and set 4 by 8 feet apart in open ground, every precaution being taken to avoid injury to the roots. Those not transplanted yielded more than those once transplanted, while those twice transplanted yielded least. The total for 10 plants with each treatment during a 3-year period was, 1,175, 1,131, and 1,001 pounds, respectively.

TomNJ

I believe most of the discussion on transplanting causing a more fibrous root system is based on a study conducted in 1927.

While I agree that particular study is often referred to because of the photos it contains, it is far from the only research available on the issue. And the correlation between fibrous feeder root development and overall plant health, disease tolerance, and subsequent production is also well documented - be it with tomatoes or any other plant.

Commercial growers and those in the nursery business will readily attest to the benefits of transplantation. But to each his own. What works for one may not work for another.

Dave

The biggest problem to me has nothing to do with root system development. It has to do with water. If you start with big containers indoors, water will stagnate in them, because there will be no root system to draw the water out.
When you first start seeds, you need small containers that can be kept moist before the seed germinates. Once the seed germinates, the soil must be able to dry out somewhat. With a tiny seedling there is only a small root system of course and this can only take up so much water. If it can't remove most of the water, Damping Off will follow, and that leads seedling death.
I keep my seedlings in the seedling trays until they get a few inches high, and then transfer to small pots which the roots will fill in short time. Again to keep from having too moist of a potting medium. This is done at about the same time as I take them in and out of the garage to harden them off. This past season, spring was cold and the wettest on record here. For the first time ever, I had a horrible time with my seedlings. I lost some from being too wet because even in the small pots, they could'nt grow roots faster than the excess water that they were exposed to.
From that point, once hardened off and the weather is decent, they go into the ground or some go into large pots. Once the weather is nice, planting from the small pot a large final pot of choice, does not seem to affect the seedling. The sun and outdoor air keep the potting mix from staying too damp and the roots at this time quickly fill the pot.
Remy

What about starting seeds in a large pot as you are doing, but only filling the pot 2/3 of the way up at first. Then, as the seedling grows, add more soil to the pot to bury the stems. Seems like this would accomplish the same thing (eg strong root system) without stressing the seedling by digging and replanting.

It sounds like from what others have said that this may work if the tap root becomes damaged when it hits the bottom of the container, which should stimulate a fibrous root system to begin to develop at that time if they are correct.

Tom,
That is great that you've never had damping off, unfortunately many people do, or we would not know what damping off is.
I've never had a problem with it myself inside, and I've grown them for 20 years. Of course this past spring with being so incredible wet and cool here, as I said before, caused me to lose some potted up seedlings outside. So I know that even with knowing what you are doing, it is possible to have failure due to small root systems and damp soil that can not dry out quick enough.
Remy

I use bigger pots to start just water around the base of the seedling up to about a inch or two away from the stem or water with a spray bottle till the girls get a bit bigger

"I use the bigger pots to start just water around the base of the seedling up to about a inch or two away from the stem or water with a spray bottle till the girls get a bit bigger"

This sounds like what I've done intuitively to not knock over early germination seedlings.

I'm wondering if some of the practice of starting small then potting out arose simply from being able thus to start many plants in a limited size, but protected, space, and be able to plant them outside once weather was fairly guaranteed to remain temperate at worst.