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Of course I know that some tomato varieties yield bigger tomatoes than others, but on a given plant, some of the fruits are bigger and some are smaller, so I am just wondering: Besides variety, what factors determine the size of a tomato (fruit)?

E.g.: Abundant water before they start to turn color swells them up? Abundant sun makes them ripen too fast before they get big? Hot weather? Fruit late in the season bigger or maybe smaller? Too many tomatoes on the plant makes them smaller? I'm sure there are plenty other possible reasons as well.

  • Lorel, there is no such thing as too much sun for tomatoes or any flowering or fruiting plant. There is the problem of too much heat. Once the temperature is in the upper 80 degrees F and 90's the plant's factories, photosynthesis shuts down. The plant stops growing and begins to reserve it's energy in an effort to survive. Removing some flowers/fruit will definitely send that energy back into the plant and fruit. Lots of sun will not make fruit ripen before their time, only too much heat or other environmental constraints that tell the plant it better make seed now not later. – stormy May 23 '17 at 21:34
  • @stormy Don't you think that you are splitting hair here? The OP obviously means 'heat' when he says 'abundant sun'. – VividD Dec 12 '17 at 17:14
  • @VividD, not really: heat and sun really are 2 different things.... Well, maybe a little unusual to have heat without sun, but sun-without-[excessive]heat is pretty common, at least in my yard. – Lorel C. Dec 12 '17 at 17:29
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This is a very open question, and I like to see other answers.

As you wrote, variety is the main driver. Cold weather and dry soil could stop the growth of a tomato.

Position is also a determinant. Usually you have several tomatoes on a stem, so the first being pollinates will have some advantage and take more resource than other tomatoes. But varieties will determines if the difference is huge or not. By removing some tomatoes, you can make the other one (often) larger. On some extend this is true also for the size on tomatoes on other stems.

A lot of water could increase quickly the size of a tomato. But then you will see it on the tomato skin, you will see scar on skin: the skin was not able to adapt so quickly.

I think also how it is pollinated could determine the size. I don't know it in tomatoes, but on other fruits, the number of seeds (so how successfully was pollinated) determines the size of fruit.

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    Your last point, how thoroughly it is pollinated, is very interesting. I was trying to think of some other fruits, and how variable their size is (on any particular plant). Most apricots, oranges, avocados, raspberries from a given plant all seem to be pretty close to a consistent size (with a few exceptional individuals either way). But strawberries & tomatoes are just all over the map, size-wise. Those last 2 back up your idea of multiseededness=>size variability. Not sure abt. oranges ... & also peaches, with their 1 seed, can have quite a range of sizes on a particular tree. – Lorel C. May 21 '17 at 13:45
  • To be pollinated or not is like a little pregnant or not. Will have no effect on the size of fruit. Will however have effect for the new genetics of the seed produced. The OP is asking how to maximize the size of tomatoes within its own genetics via environmental, manual constraints. This is all about managing energy, not genetics. – stormy May 21 '17 at 21:10
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    @stormy: It seems that in wine grapes, the size depend on pollination. There is (IIRC) only 4 seeds, but if there were a bad (rain) weather during flowering, the grapes will be smaller and with less seeds. And if I remember correctly, this is true also on other fruits (strawberries, raspberry, etc). And genetics has a lot to do. Common fruit varieties are polyploids (in order to have larger fruits), so quantities of proteins and hormones affect the size. – Giacomo Catenazzi May 22 '17 at 8:18
  • I have a problem with this concept. If you are right I've got some major work to do. When pollination occurs those flowers are impregnated with new genetic material. Can't imagine that there are that many different genetics promoting size with grapes that could make a change. I shouldn't translate ever human stuff to plants but because this is you, Giacomo, sigh, I am going to have to learn something new. Gee, thanks!! Grins!! But for real, I love learning I've got a hole in what I know. If this is true then it has to translate for other plants not just grapes. I shall let you know! – stormy May 22 '17 at 22:21
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    @stormy: Think again about women. In any case, fruits or seed could be started in advance (like eggs). Think about inferior-ovary flowers. Sometime this ovary (and future fruit) is already noticeable. Just if they receive the genetic material, it will continue to growth. On some plants this will create more hormones which will create large fruits. [Female periods are similar: material is prepared in advance and discarded if no fertilized egg. Much of this is controlled outside egg genes] – Giacomo Catenazzi May 23 '17 at 19:38
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While much good information has been provided, some is incorrect or partially correct. Tomatoes are primarily self pollinating, and not just to the extent of pollen being transferred between flowers. Pollination can and does occur by the pollen of one flower fertilizing the ovule within the same flower and can occur prior to the actual blooming of the flower. This is why tomato cultivars can be maintained relatively reliably without extensive manual labor. Flowers can be bagged prior to blooming which prevents cross pollination as well as pollinators from assisting fertilization. Pollinators DO assist in fruit formation as they increase fertilization by vibrating ( a bee's buzzing sound) but it is not required. Fruit size is however related to level of fertilization. Unlike what was said re: " a little bit pregnant" being impossible it is in fact exactly the case in tomatoes that more fertilization yields bigger fruit. Each fruit contains many seeds. The seeds influence fruit development by altering plant growth regulators in the fruit. Evidence of this is that sterile plants (due to chromosome imbalance fruit much more poorly and fruit size is much smaller corresponding to the number of seeds. This is also why tomato bloom set chemical sprays work. They mimic the growth regulators the seeds would normally produce in otherwise poorly fertilized situations

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In addition to Giacomo's good answer, I'd like to mention that plants need a certain amount (sum) of light and temperature. If a fruit develops when there's more light each day and the temperature is higher, it needs fewer days compared to a fruit that started to develop when the light hours each day were fewer and colder. The fruit that developed in a longer time period has the opportunity of a higher intake of water, since you can only water this much before the skin of the fruit breaks.

Fruits of different sizes on the same plant might form on allogamous species because of pollination from another plant with compatible pollen.

Different fruit size might also happen in autogamous plants because self-pollination leads to expression of recessive genes.

The tomatoes and strawberries you mention are likely to be polyploids, meaning they have more cromosomes' sets, and therefore more seeds and larger size.

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Potassium is highly implicated in fruit size. I've used a lot of potassium for the last couple years, and I can say that it does seem to help for most varieties in my growing conditions (but not all; Omar's Lebanese, and Marianna's Peace are a couple examples that didn't seem to benefit in my growing conditions).

Potassium can help plants absorb water. Copper may also help fruit absorb water, but I don't know if it increases fruit size. Potassium may change the flavor and/or texture, however.

Extra nitrogen, however, may (in some situations) actually increase the number of fruits that set, while not increasing the total weight of the combined fruits, according to a study I read about (which seems to say that the fruits would be more plentiful, but smaller). Excess nitrogen in other conditions may lead to few fruits and excess foliage, as is more commonly thought.

It's not all about potassium and nitrogen, though. Clearly, from my experience, there are other factors, although it's difficult to pinpoint them all. I personally believe that acclimatization to the soil and growing conditions over both the season and generations can help. Sometimes, fruits that set later on are considerably larger than the first fruits (this may have to do with the plant maturity, and how acclimatized it is).

Some people have tricks that pertain to pruning, removing other fruits, and soil amendments to increase fruit production and/or size. This is especially true with some varieties.

I don't know about for tomatoes, but some people say that micorrhizae helps in growing giant watermelons.

Hybridization between two varieties can sometimes lead to fruits larger than either parent variety.

Container size can also influence fruit size.

  • Oh I think I wholeheartedly agree, Shule. Once one has a seed, the genetics are fixed. Manipulating the environment is the only way I know to enhance size within a fixed genetic parameter. All plants need soil organisms to up take the nutrients plants have to have to make their own food by capturing the energy of light. All plants need the chemicals with which to do photosynthesis. To grow large fruits those chemicals, proper watering, everything that plant needs to produce enough energy to produce large fruits has to be addressed. Without mycorrhizae and bacterias and the macro.... – stormy May 23 '17 at 21:17
  • organisms in the soil, plants are unable to use NPor K for photosynthesis. Gotta mention the dozen or so micro chemicals (micronutrients) that are equally important. Container size is important for the plant's health which in turn creates fruit that that plant is genetically programmed to make. As long as there is enough light, regular water not too much not too little, enough chemicals so the plant is able to do photosynthesis and make its own food and energy and seed and fruit, excellent drainage, no disease or insect vectors...one should be able to realize the fruits predetermined by.... – stormy May 23 '17 at 21:23
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    ....genetics. Someone just mentioned potassium to grow larger fruit. It is not more potassium it is rather higher potassium and phosphorus in relation to nitrogen that enhances reproductive growth. These discussions are incredibly beneficial to both the OP and for us gardeners! To test what each of us knows in our own world of knowledge and experience is so important for us gardeners to grow and be better at teaching this knowledge. This is why I am here. Love teaching but I have to know that what I teach is correct down to the nitty gritty! Love these discussions! – stormy May 23 '17 at 21:28
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No one has mentioned fertilizer. I guess because 'fertilizer' has somehow gotten a very bad reputation for being artificial no matter synthetically or organically produced.

Plants have to have very precise amounts of chemicals with which they use to make their own food, roots, photosynthetic growth that makes food for the plants, seed production and obviously fruit production. Soil does not come with these chemicals/fertilizers naturally. In self sustained ecosystems, there is little to none of these chemicals in the soil until something dies. That helps to maintain the system and limit population and subsequent needs of these chemicals.

The chemicals necessary for photosynthesis which makes the food for the plants to use as energy, also are responsible for the size and vigor of the plants and the size of the reproductive parts of plants; such as size of tomatoes.

The amounts of fertilizer necessary are found through soil tests and the ability to recognize the needs of your plants by how they look. And the sooner the better. When someone gets to the point of 'knowing' without soil tests what their plants need via experience and remembering the formulations already added, what kinds of organic material that has been added, what the symptoms look like by the plants showing deficit, excess of chemicals (nutrients) one is able to manage the chemistry of their soils to optimize vigor and larger fruits.

The manual process of taking reproductive growth off the plants; flowers and fruit/seeds is an important way to add more energy back into the plant and its vegetative growth. Cutting flowers off even before they open is excellent. You enable more energy into your plant to get larger, more vigorous, store more food/energy with which to produce even more flowers (especially annuals). To get more fruit/larger fruit you allow the flowers to grow into seed/fruit and select certain fruits you want to grow large. If you have a lot of flowers thus a lot of fruit you'll get smaller fruit if there isn't enough room to grow larger and if there isn't enough chemistry to put energy made by the plant into larger fruits. Taking off flowers and/or fruits will send that energy into the seed making/fruits and vegetative growth that are left.

If there are the proper chemicals, lighting and water to continue the photosynthesis you won't have to worry about pruning reproductive growth. Regular watering, regular temperatures, regular vigilance with chemicals will make fruits the size they were genetically designed to make.

The problem is, just a wee bit too much fertilizer will kill your plants. The pH of the soil has the ability to remove chemicals from access by your plants. The temperatures do the same thing. Some of the chemicals necessary if added in too large amounts or if deficient will also cause some chemicals to not be accessible to plants. If the soil has no life; mycorrhize, bacterias necessary for plant roots to up take chemicals that are in the soil, that will hamper plants as well to be able to access and up take necessary chemicals to make food/energy with which to make more vegetative growth or reproductive growth.

In addition, one of the most important concepts to produce reproductive growth versus vegetative growth is the proportions of chemicals added. If for instance you add a fertilizer with a formulation of 7-6-4, you will promote vegetative growth. If you add a fertilizer with a formulation of 10-12-14 you will enhance reproductive growth and the propensity to be able to get larger fruits. Too much Nitrogen in relation to the Phosphorus and Potassium percentages will enhance vegetative or leafy growth. Reproductive growth will suffer.

Great for salad greens, kale, rhubarb but if you want reproductive growth; raspberries, tomatoes, squash, cucumbers you need to watch carefully that the Nitrogen is less than the other two macro chemicals.

These are chemicals not nutrients or food. That is why I try to not use the word 'nutrient' as people think of nutrients in their own terms of food/fuel. Too much food for us just makes us fat not dead. For plants, there is an optimum amount of chemistry; a little too much of one chemical causes symptoms of excess, too little causes symptoms of deficiency and just a little too much of a few chemicals will most certainly kill plants.

Fertilizing is an important science we humans have to learn and understand. Stable ecosystems where we humans have not had the chance to disturb are so finely tuned there is just enough for the health of plants until something is allowed to die to be added back into the ecosystem for use by that ecosystem.

We humans go in and remove the biomass of these ecosystems and that soil will be deplete for the needs of plants we want to grow or for new plants in that ecosystem. The rainforest for instance. We burn and cut down the rainforest and that land is worthless for crops. Maybe the first year there might be enough chemistry to grow one crop but that is it. Most people have heard this but this is true every time we remove the forests and expect yummy soil to grow our crops. We humans are actually mining the chemistry of that soil by removing the biomass.

We have to learn how to add chemicals back into the soil so that we are able to grow crops for ourselves to eat. If we do not understand the chemistry in our soils and do not add fertilizer because some article says that fertilizer is bad we have misunderstood the entire biological/botanical/mineral parts of the system and we will not be able to grow anything with success...

Soils are an amazing science. The only way that has been found to improve our soil texture/structure/organics to enhance botanical growth is by the addition of already decomposed organic matter. To decompose organic matter organisms that are in charge of decomposition need lots of nitrogen. Once they have done their job by decomposing something newly dead, only then is that organic matter now decomposed available for use by micro and macro organisms in the soil to eat for energy. While organic material is being decomposed all the other organisms go to sleep, dormancy or die. Once the decomposers have decomposed organic matter there is then 'food' for the micro and macro organisms to wake up and reproduce. Plants need to have these micro and macro organisms to uptake chemicals they need for photosynthesis (to make food and seed). The decomposed organic matter is critical as well to improve the tilth of the soil for air, organisms and plant roots to thrive.

What is it that you are wanting to accomplish? Large tomatoes, more tomatoes, longer lasting tomato plants? What have you been using for management skills? What hasn't been working?

Hope this helps generate more questions...ask away!

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    Hi @Stormy, to answer, what am I trying to accomplish. I'm not really trying to grow mammoth gigantic tomatoes. I just have a couple tomato plants, with quite a few tomatoes on. But I noticed one of them beginning to turn a slight yellow the other day, so I think it's about to start pinkening, then reddening, and probably won't get much bigger than the current 1 and a half inch diam. I think a little small for Early Girl. – Lorel C. May 23 '17 at 21:54
  • Early girl is a nice, hefty sized tomato! Once my tomatoes start to 'pink'...they are off the vine and able to finish ripening in my kitchen. I've never found a difference between vine ripened and kitchen ripened. I dehydrate or 'sun dry' most of my tomatoes anyway. At the end of our season, I allow a few to ripen on the vine but even though I have very excellent taste buds and a great smeller, I've not found any benefit to allowing tomatoes to ripen on the vine. 1 and 1/2 inch is WAY too small for early girl. What soil are you using and what fertilizer? – stormy May 23 '17 at 22:06
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    Sorry, my computer stopped in mid-comment. Anyway, I meant to say, I am OK with little tomatoes, but just got to wondering what causes size variation - especially in tomatoes, when a lot of other fruit is pretty consistenltly the same size all over the plant [or tree]. But now I realize, of course, a lot of fruit, especially tree fruit, blossoms at all the same time, so all fruits ripening under pretty much identical conditions, environment-wise at least. So the question was just curiosity really. – Lorel C. May 23 '17 at 22:12
  • Oh I get what you are saying completely. The thing about tomatoes especially if they are indeterminate is that they are flowering constantly. I've never done anything more than give my tomatoes the best soil, proper fertilizer, lots of sun and cover them when it goes nuts with rain. I pretty much water tomatoes in pots every day. Hey, smaller tomatoes have more flavor, generally because they have less mass and more water. Trees are different. And to be a true gardener you have to be curious or you would not be a true gardener Lorel!!! This is a super question! Look at the response! – stormy May 23 '17 at 22:51

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