Table of Contents
Introduction
Planting Details
Harvest Data and Analysis
Conclusions
Introduction
When should you plant tomatoes for the optimum harvest? How should you stagger your plantings to produce a steady harvest of tomatoes for the longest interval?
Having grown tomatoes in Southern California since 1979, I had the following intuitive answers from my experience. All of the following pertains to full-size tomatoes. Cherry, grape, patio, and plum tomatoes will ripen as much as a month earlier, since it takes less time for them to grow from a pollinated flower to fruit.
First, it hardly makes any difference whether one plants tomatoes in early March or in early May. The first tomato usually ripens in late July, and full production usually begins in early August, no matter what the planting date, as long as the tomatoes are in by early May.
The reason for this is that for most varieties of tomatoes, tomato fruit will not set until the nighttime temperature is above 55° F for at least two nights in a row. Tomato flowers need to be pollinated within ~50 hours or so, or else they abort and drop off. It takes about that long for the pollen to germinate and travel down the style to fertilize the ovary in temperatures close to, but above, 55° F. The pollen takes much longer to do so in colder temperatures, so the clock runs out on the pollen if the temperature is too low.
Most years, it is simply too cold at night for fruit to set until sometime in June. The average nighttime temperature doesn't go above 55° F in Fallbrook until 30 May, and in Altadena until 11 June. So full production cannot begin until the average nighttime temperatures are consistently above 55° F.
A small number of earlier tomatoes can result from lucky early periods of heat. If that happens, fruit might set earlier and it is possible to get the first tomato by July 4 or so. However, full production would still not be expected to occur until early August.
For example, in 2004 in Fallbrook, we had two successive nights with minimum temperatures above 55° F on 8-9 March, 25-27 April, 2-4 and 16-17 May. The minimum temperatures were not consistently above 55° F until 2 June. Since for most tomato varieties it takes about two months to go from blossom fertilization to ripe fruit, a harvest might be expected as early as late June through July, if the plants had blossoms on 25 April and later. In 2004, I planted my first crop of tomatoes on 16 April, and they probably didn't have any flowers until mid-May, since I picked off the first set. I picked my first tomato on 16 July, which most likely resulted from fertilization on 16-17 May.
However, my neighbors Carl and Donna Nelson planted tomatoes in early March, which were big enough to have plenty of blossoms by 25 April. Those four nights of high minimum temperatures between 25 April and 4 May were apparently enough to produce a good crop by late June.
Second, the first planting of tomatoes is essentially finished after a month or so, and it is necessary to plant a second crop of tomatoes to extend the harvest. Unlike the first planting, it is essential to get the second crop of tomatoes in before the end of July. Planting later than July usually results in essentially no harvest.
Beginning in the year 2000, I decided to analyze the harvest quantitatively, to see if my intuition was correct, and to put numbers to the harvest times. This article presents the data and an analysis of the results.
Planting Details
In Altadena, I experimented with a number of varieties and found that only two gave good results: Early Girl and Better Boy. I tried Ace, Beefsteak, Celebrity, and San Diego, among others, and being disappointed with their production or quality. So all the results below are from those two varieties, with roughly equal numbers of each variety.
In Fallbrook, I have had the luxury of planting tomatoes in a totally new area each year. It is well known that one must allow at least a three year interval between planting tomatoes or a close relative in the same area, to minimize disease.
Each tomato bed is 4 x 8-10 feet, aligned east-west. I dig up each tomato bed once, add about an inch of home-made compost, scatter some fertilizer containing nitrogen, and dig it all in. I then plant tomatoes two feet apart, in two rows of 4-5 plants each. I place around each plant a 6.5 foot tall cage, 2 feet in diameter, made of concrete reinforcing wire, which has 6 inch openings.
I usually plant cherry tomatoes in the corner positions, to draw birds to them instead of the big tomatoes.
I plant from 6 packs, after breaking up the outside of the root ball, and stripping the lowermost leaves from each plant. I plant them such that the remaining leaves are just above the ground surface, which has two advantages. First, it places the root ball deeper into the ground, where the moisture is more constant. Second, it allows for a more vigorous root system since roots can develop all along the stem. Roots are the fundamental limiting factor in total tomato production, which is why plants in pots never grow as big or produce as many tomatoes as plants in the ground.
I water the plants every couple of days when they are young, gradually increasing the interval to a deep watering once a week. I give them an extra watering if the high temperature is near 100° F.
I pick off the first flower cluster on each plant, so that the plants can devote their initial attention toward growing leaves, the energy source for producing tomatoes, rather than growing flowers or tomatoes when they don't have adequate resources to both grow leaves and tomatoes. I never prune the plants in any way, since all previous research has shown that the more leaves the plant has, the more tomatoes it produces.
It is bizarre that some people think that you need to pinch off the axillary leafy growth "so that the plant can devote its energy to the fruit". Where do these people think the energy is coming from? The only reason for ever pruning a tomato plant is if one doesn't have room for the full growth of the plant, usually because one is tying the plant to a pole, and one will then suffer the consequences of reduced production and increased sun-scalding of the fruit.
The tomato cages make maintenance of the plant simple as it grows. You simply poke back into the cage any shoots that try to leave the cage. No tying is necessary. The 6 inch mesh is plenty big enough to reach through and pull out even the biggest tomatoes.
I never spray the plants with anything, and I don't fertilize them after they are planted. The only pest strategy I use is to hand-pick tomato hornworms if they appear, and gopher control nearby. My tomatoes are always surrounded by enough bare dirt that snails don't get into them. Occasionally rabbits have nibbled some of the lower foliage.
Harvest Data and Analysis
Each harvest day, I recorded the number of equivalent-full-sized tomatoes I picked. That is, I consider a full-sized tomato to be about 3 inches in diameter, equivalent to about a half pound in weight or perhaps a bit less. I simply made a quick count of my harvest, correcting for the size of smaller tomatoes in a rough way.
This correction to equivalent-full-sized tomatoes is usually only significant for the last tomatoes to be harvested, which are generally much smaller than the main crop.
The only exception to the correction may have been in my earliest data for the year 2000, when I may simply have counted the number of tomatoes, and not attempted to standardize the numbers.
I counted only harvestable tomatoes that didn't rot before I ate them. Late in the year, tomatoes would sometimes rot in a few days before I ate them, so I didn't count those as a harvest. I have notes about this happening only for one year, so this doesn't happen often. In contrast, tomatoes picked earlier in the year last for weeks on the kitchen counter without deterioration.
I divided the harvested tomatoes by the total number of plants that I planted, even if some of them died before producing many tomatoes. That is, occasionally one or two tomato plants out of eight will simply die from disease or gophers, and I did not correct the data for that loss. My aim was to record what I actually harvested from plants I planted, not the theoretical best that a single plant could do if it didn't have any problems.
The detailed cumulative harvest data, in terms of tomatoes per plant, is shown in the following plot:
Each curve is labeled by the year of planting, with the first crop planted in a year denoted with - 1, and the second crop with - 2.
As noted above, it is possible that the top green curve, from the year 2000, is artificially high because I did not correct it to equivalent-full-sized tomatoes. If that it the case, it might actually top out at about 50 tomatoes per plant like the next two curves.
The plot confirms my intuition about the relative insignificance of the date of the first planting. There is almost a month variation in the plant dates for the first three curves, from 18 April to 11 May, yet there is a remarkable consistency in the time and amount of production. Tomatoes start to roll in at full production around 1 August, and end full production around 1 September, with a total production of about 50 equivalent-full-sized tomatoes per plant, averaged over all plants.
Those data are summarized versus planting date in the following two plots:
In the plots above, I didn't include the three plantings with pathetic yields, since the small numbers from those plantings would cause relatively larger errors in these plots.
The first plot above uses actual calendar dates; the second plot converts those dates to Days Since Planting, since that is an advertised character of each tomato variety.
The advertised dates for my two varieties are: Early Girl, 62 days; Better Boy, 72 days. I didn't keep track of which variety each harvest was from, but it is quite clear that those dates are complete fictions for the earliest plantings, where the first tomato typically comes in more like 90 days. Again, this is undoubtedly due to the nighttime temperatures being too cool in the spring for fruit set.
My observed ~70 days for later plantings is more in line with the Better Boy advertised number, and it is possible that I could have obtained something close to the 62 days for Early Girl if I hadn't picked off the first flowers.
The second plot above shows dramatically the decrease in total tomato production lifetime from later plantings. The first plot gives the date of the last harvest as being early- to mid-October.
The data also confirm the value of planting a second crop, which begins full production around 1 September, just as the first crop is ending full production, and ends full production around a month later.
The plot also reveals something that totally surprised me. The second crop is strongly variable, and is sometimes almost non-existent! Planting dates varied from 1 June to 15 July, and the total production was always significantly less than the first crop. The total crop ranged from a pathetic four equivalent-full-sized tomatoes per plant, to 38 equivalent-full-sized tomatoes per plant.
The correlation of total yield to planting date is shown in the next plot:
Three out of five plantings from 1 June and later produced a pathetic seven equivalent-full-sized tomatoes per plant or less. Two of the five plantings produced respectable yields of 32-38 equivalent-full-sized tomatoes per plant.
The most likely source of this variability is disease and pests, probably fusarium wilt and nematodes. USDA studies have concluded that fusarium wilt is one of the most prevalent and damaging diseases of tomatoes across the U.S. (Source: All About Tomatoes, West edition, Ortho Books, 1976, p. 59). This is why it is important to plant wilt- and nematode-resistant varieties, such as Early Girl (V) and Better Boy (VFN), where the letters indicate which disease it is resistant to. (V = verticillium wilt, favored by temperatures of 70-75°; F = fusarium wilt, favored by temperatures of 80-90°; N = nematodes).
In nearly every place I have grown tomatoes, except Fallbrook, my tomatoes would typically have a 2 foot height layer of green foliage that traveled up the plant. Foliage below that layer would die from one of these diseases. I was fortunate in Fallbrook having soil with a much lower level of these diseases, and having enough acreage to plant in a totally new area each year. In Fallbrook, my plants stay green along their full height of ~6 feet, until later in the year when the diseases begin to take their toll.
I speculate that the typical high temperatures of 90° in July, August and September take their toll on my second planting by encouraging fusarium wilt and/or nematodes, reducing the yields.
Another possible source for the decline in production is the signal sent to the plants by the declining temperatures and sunlight in the fall. Although indeterminate tomatoes can theoretically live forever if not killed by frost, the tomato plant clearly has some genes in it that receive signals to stop growing and ripen your fruit before you die. These genes are clearly expressed in determinate varieties, which grow to a certain height, then ripen all their tomatoes at once, and then die. By late September, the hours of nighttime have already equaled the hours of daytime, so the signal is quite strong.
The declining sunlight level itself is another possible cause for the decline in production. To investigate this, I made a simple model for the amount of solar energy absorbed by a plant over its lifetime, depending on planting date. I assumed the amount of absorbed solar energy is simply proportion to the leaf area of a plant, which was proportional to the following model:
| Month | Month Example | Leaf Area |
|---|---|---|
| 1 | Apr | 0.1 |
| 2 | May | 1.0 |
| 3 | Jun | 3.0 |
| 4 | Jul | 6.0 |
| 5 | Aug | 6.0 |
That is, I considered the mature plant to be 6 feet high, with its foliage filling my tomato cage. At month 3, I considered it to be 3 feet high. At month 2, I considered it to be 1 foot high, but with the foliage filling the diameter of the cage. At month 1, I took it to be about 10% of its leaf area at month 2. By month 6, I assumed the plant was so weakened by disease or old age that it ended production.
In the example above, I planted the model plant in April. In the model, I varied the planting date from March to October.
Although the solar energy absorbed also depends on sun angle, it actually doesn't change much with sun angle for my plantings, as shown by this simple model. First, consider the sun to be directly overhead on my two rows of plants in a 4 feet x 8 feet bed. The absorbed sunlight would be the solar constant at the earth's surface times 32 square feet. Second, consider the opposite extreme, with the sun shining horizontally from the south. My plants would absorb the solar constant at the earth's surface times an atmospheric absorption factor (to account for the longer path through the atmosphere) times 8 feet x 6 feet (the height) of sunlight. If the atmospheric absorption factor is near 2/3, this is exactly the same energy as when the sun was overhead. So I ignored sun angle in my simple model.
I then obtained the monthly energy input by multiplying the plant leaf area by the total number of hours of sunlight each month. Finally, I computed the total energy input for each plant by summing the monthly energy input over the lifetime of each plant.
The result is shown in the pink curve in the plot above. The total amount of energy available to each plant is fairly constant for planting dates from March through July, but then declines precipitously beyond then since the plants don't mature until well after the peak sunlight of late June.
It is clear from this simple model that the decline in tomato production for the second crop has very little to do with available sunlight. If one could eliminate disease, it is quite likely that the harvest from later plantings would be significantly better on average.
However, later harvests are disappointing in tomato texture and flavor. Tomatoes, like many tropical and subtropical fruits, do not like cold. Their flavor begins to decline below 54° F, which is why you should never put whole tomatoes in the refrigerator. Essentially what happens is that cell damage begins to occur at colder temperatures, resulting in mushy tomatoes, and their sugars are either consumed or degraded as the contents of the cell spill out into a mess within the tomato, resulting in a tasteless tomato.
The average Fallbrook low temperature is below 54° after 29 October, so tomatoes harvested from November through May are quite disappointing. In a number of years in Altadena, I have had tomato plants that continue to produce fruit through January or so, but they were not taste treats to consume.
Conclusions
- For the maximum tomato harvest time, tomatoes should be planted in at least two crops.
- The first crop can be planted anywhere between 1 March and ~15 May, and in most years, will result in full production between 1 August and 1 September, independent of the planting date. However, early warm nighttime temperatures might produce a small number of earlier tomatoes.
- Don't believe in any correlation between the advertised number of days to first tomatoes with what you will observe in Southern California for the first crop.
- If you desire early tomatoes, either hope for a hot spell in spring, grow your early tomatoes in a heated greenhouse, or plant varieties that will set fruit in nighttime temperatures below 55°.
- The timing of the second crop is tricky. In order to have full production between 1 September and ~1 October, it is necessary to plant the second crop between ~1 and ~15 June. Although the risk of low production is quite high for plantings at that time, one can make up for that by simply planting more tomato plants.
- Planting after ~1 July probably guarantees a very low yield of tomatoes, and is not worth the effort of planting and caring for them.
- The number of plants required to satisfy a given demand for tomatoes can be calculated for the first crop by assuming a yield of about 50 good-sized tomatoes per plant, with essentially all of them coming uniformly over a month.