Ask a Question forum: Container Tomato Plants

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Name: Jackie Svorinich
Jul 7, 2016 6:06 AM CST
I have a Tomato Plant growing in a window box out on my balcony. It has flowers but the bottom leaves are turning yellow.Any idea why? Thank You!!! Happy Gardening! ๐Ÿ…
Name: Robyn
Minnesota (Zone 4a)
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Jul 7, 2016 8:18 AM CST
Well, I was growing a tomato in what I thought was pre-fertilized potting soil. I don't remember exactly, but I think I had already used that soil with another plant that I had transferred out but anyway, the plant started getting really yellow! It started this almost neon lime green. The plant was hungry! I mixed up a used coffee grounds and old coffee sludge and watered it with that, put some peat on top of the dirt and mixed that in. I've also since given it a little bit of actual fertilizer though, not much. I can always give it more but I don't want to overdo it. It is now greening up and doing much better! Looks like it needed that nitrogen! I also planted fenugreek seeds in the pot with it, they are nitrogen fixers plus I get leaves that smell like maple syrup for tea!

A few weeks after being moved to this pot:
Thumb of 2016-07-07/robynanne/504f10
Another weekish after, when I noticed a problem:
Thumb of 2016-07-07/robynanne/c32422
A few days after feeding it, the yellow finally peaked:
Thumb of 2016-07-07/robynanne/3cf7c7
A few days later, PHEW! It was getting better:
Thumb of 2016-07-07/robynanne/07ce5f

Other things that can cause yellow: too much water or not enough water, or I read that over fertilizing can cause yellow as it can kill the roots with too much salt and thus the plant can't get nutrients. I think that is unusual though since most of the images for over fertilized tomatoes had bluish leathery leaves.
Name: Rita
North Shore, Long Island, NY
Zone 6B
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Jul 7, 2016 6:30 PM CST
Jaxs said:I have a Tomato Plant growing in a window box out on my balcony. It has flowers but the bottom leaves are turning yellow.Any idea why? Thank You!!! Happy Gardening! ๐Ÿ…

I hope you keep it well watered. Unless it is a container variety and a small plant you window box probably aren't big enough for a tomato.

As for the bottom yellowing leaves, that is not uncommon. Tomato plants are very prone to fungal diseases. Cut off and discard those yellow leaves in the trash, Not the compost pile.
Name: Rick Corey
Everett WA 98204 (Zone 8a)
Sunset Zone 5. Koppen Csb. Eco 2f
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Jul 7, 2016 7:37 PM CST
Hi Jackie. Welcome to NGA!

I agree with all of Robyn's and Rita's points.

First make sure that the container has lots of drain holes and they are not plugged, and water runs out a little while afteryou water them.

1. if the potting mix has dried out and shrunk, "watering" might actually only let the water flow AROUND the potting mix without wetting it. If that's happening, the plant will also die of thirst, so that's probably not it.

2. If the drain holes clog up or there are too few or they are too small, OR the potting mix is too fine and holds too much water to let any air in, the mix would usually be soggy and the roots would be dieing and rotting. It's probably not that unless the plants start getting much worse. But if the soil IS soggy and not well aerated, you will have a variety of problems including root rot and salinization of the potting mix (salts accumulate if no water flows THROUGH and out)..

3. If nothing else was wrong except the container was too small for that variety, you would be likely to see exactly this: lower leaves yellowing. Try a soluble fertilizer like Miracl-Gro, or if there is a fast-acting high-N organic alternative, try that. Since the plant can't make as many roots as it needs in a small container, make sure the plant CAN still get enough minerals from that small container.

I think that 3-5 gallons is the smallest you would want for tomatoes, with determinate cherry varieties OK with 3 gallons, but mainstream indeterminate tomatoes would want at least 5 gallons.

4. if nothing else is wrong AND your window box is big enough, maybe the soil in it has too little nitrogen (and maybe not enough of some other plant nutrients). Low-N looks EXACTLY like you describe: yellowing lower leaves. (But other things could cause that, too.)

Again, try soluble chemical fertilizer or an organic alternative (if any of those are fast-acting enough and have enough N).

The thing to be careful of with soluble fertilizer is that it IS potent. It's very easy to use too much, and plants often just die when you do that. So read the instructions and start out with about half that much, applied once per week until they get better or die.

And remember that water "should" come out the drain holes several minutes after you water heavily. That carries away excess nutrients and salts, preventing saline buildup that can kill plants. When I say "water should come out", I really mean that the potting mix "should" be open enough to allow drainage.

Water needs to drain out so air can diffuse in.

Air diffuses literally 10,000 times faster through air-filled voids and pores, compared to diffusion rates through water-filled voids and pores. When the mix is so fine-textured that most voids and pores aren't much bigger than a "capillary distance", most pores and voids grab any available water and hold it in a firm capillary grip. Now air is excluded until the plant drinks almost all the water in the container. Roots quickly die and rot.

When the mix is coarse-textured enough that many gaps between solid particles are BIGGER than a capillary distance, gravity can pull some water OUT of those bigger voids. Now part of those voids are air-filled, and oxygen can diffuse through them 10,000 times faster. Roots get oxygen and live.

[Last edited by RickCorey - Jul 8, 2016 1:44 PM (+)]
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Name: Sandy B.
Ford River, Michigan UP (Zone 4b)
(Zone 4b-maybe 5a)
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Jul 7, 2016 7:57 PM CST
Welcome to NGA, Jackie ( @Jaxs ) !

A window box wouldn't be likely to have enough depth to keep a tomato plant happy; you might want to consider moving it to a deep pot that holds about 5 gallons of potting soil. (Can you tell us what variety of tomato you have growing?)
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