Tomato seedlings - environmental variables that affect growth

I have a pretty good tomato yields, but one thing bothers me every year. My tomatoes grow very tall, and they set fruits only high above the ground and with quite large distance between fruit clusters. I grow my tomatoes seedlings inside, I plant them out outside after last frost, and they grow outside, outgrowing trellises. I would like to have them more "compact", so my question is: what environmental variables (putting tomato variety aside) determines their height and the distance between flower / fruit clusters?

Richard Gorny wrote:what environmental variables (putting tomato variety aside) determines their height and the distance between flower / fruit clusters?

It seems to me, like genetics matters more than anything else in determining the distance between nodes, and how frequently a node is a flower cluster. If nodes are too far apart for your liking, it seems much easier to change which varieties you are growing than it is to change your weather, ecosystem, soil, habits, etc...

With that out of the way, low light level tend to encourage etiolation in many plant species. Increasing light levels leads to stockier plants. Increasing water tends toward producing lanky plants.

Length of the dark period...
Plant later, plants will be compacter than those planted in spring

If you have a lot of nitrogen in your soil your plants will be taller. I can't tell you in numbers how much is too much. However, from my experience, if you spread two inches of horse manure and then when planting the tomato seedlings into an over size hole you mix in another few spade fulls; your tomato plants will be 12 to 14 ft tall. That's over 4 meters tall. You will get the same number of tomatoes as you normally get. If you've been getting blight of some kind the tomatoes will grow so fast that they just out grow the blight.

If you're using a chemical fertilizer, which I have never done, you might consider using less or using a fertilizer with a lower first number. In fertilizers where NPK  expresses the ratios the first number is the amount of nitrogen.

In my case I had actually used mushroom manure which is composted and steamed and was used to grow two crops of mushrooms. Actual horse manure would have been even more extreme. Had I used this over barren clay it might not have been an excessive amount?? It's said that if you use even more nitrogen; that you can have a situation where you get none to few tomatoes. Myself, I've never tested that.

Yes, I had the same issue with tomato seedlings from a nursery - probably not an heirloom variety.

Genetics aside, it's also likely the ground I planted them in had too much fertiliser. Tomatoes will grow 'leggy' with lots of greenery and fewer fruit with too much nutrient. Need to keep them off the (N) at certain times and provide higher levels of (P) and (K).

Also, I understand it's temperature not day length that ripens tomatoes.

I share Joseph's experiences. Right now I'm growing two varieties of tomato, and one of them has fruits tightly spaced about 4" apart mostly close to the ground. The other is twice as high with fruits spaced about a foot apart. I'd say both have a similar amount of fruits. In my experience tomato plants are some of the most resilient in terms of starting. If the seedlings end up too leggy, I  plant them deeper in the ground and they bounce right back. It doesn't seem to matter how badly the seedlings are abused, once they're in the ground and treated properly, they seem to find their own way.

In my humble opinion it's neither temperature, nor length of daylight. This year in western Pennsylvania, USA we had a late spring. We're now having late harvests. Peaches, tomatoes all are late. My tomatoes were safely in the south facing window except that they were out in the sun on nice warm days getting hardened off. So the tomatoes didn't really get the effects of the late spring. But they seem to know. Don't ask me how. Soil temperature?? We're having warm temperatures now, for the time of season. I'd say we had a somewhat cool summer, and definitely a very wet year. Wet spring, wet summer. Many cloudy days, through much of the summer.

Seems like all my life I heard conflicting opinions on what one needs to ripen tomatoes. Some say you need warm nights, some say you need cool nights. Both sound plausible.

I'm growing all heirloom tomatoes. 10 different beefsteaks, plus a big yellow and a yellow pear. One just came ripe yesterday, the same variety (Dester) a different plant came ripe today, Each was planted at the extremes of my planting schedule. Two have still not ripened. Pink Brandywine (they're actually red) and Pink Ponderosa. I'm getting taste results all over the place. Beefsteaks too watery, extremely acidic, high in flavor and aromatic.

Many thanks everyone for your replies, a lot of valuable information here.

I observe that commercially grown seedling that are being sold in a gardening center nearby differ from the ones I grow from seeds of (presumably) same variety.

I have tried to grow my seedling in natural light, as well as under quite powerful LED / fluorescent lights indoors, always getting same effect - seedlings reach lights very soon and I have to rise the lights to avoid burning plants.

From seeds sown in the first week of April, I have 2 feet plus tall plants by May 15th (the date of last frost). I bury them deeply, but it is not helping - first fruits form very high on the plants, while other gardeners have them almost touching ground.

During the season the plants grow vigorously, and they set flowers and fruits very far apart. No matter what is location in my backyard (and that determines amount of direct sun) and method of growing (I grow them in the soil, in grow bags and in strawbales).

Since I use my own potting mix for seedlings (based on what you guys say above), is it possible that it is too rich in nitrogen? Shall I reduce amount of compost I add to the mix?

Finally, if nothing else helps, what varieties would you recommend that sets nodes densely and close to the ground?

From When my seedlings are just comming up to when they are planted outside I get a page of paper and brush over them, I blow on them and set them by a fan or open breezy window sometimes. This is because I live by the sea and its usually mild but we get some strong storms blow in and simulating the windy conditions from germination helps them grow stockier and avoids leggy easily damaged plants.
This is just my method I've come up with after just a few years of growing but it might also work for you

I take mine outside every day if weather allows, for at least one hour, sometimes for half a day.