Basic horticulture of growing tomatoes

The common tomato or scientifically classified as Solanaceae Lycopersicon lycopersicum (L) Karst. ex Farw. is one of the most popular vegetables planted both by commercial farmers, small scale farmers and home gardeners.  It is actually a pleasure to grow tomatoes as there are so many varieties available today that marketing opportunities are abundant.  For the small scale farmer, especially subsistence farmers, the common determinate open pollinated varieties form part of their food basket, while home owners and commercial farmers tend to be more adventurous and variety selection.


Climatic requirements of growing tomatoes

bush-tomatoes-876963_1920The tomato plant is a sub-tropical plant and will not tolerate frost.  By sub-tropical I mean it is a warm season crop. It is grown in the summer. This means that it will grow optimally between ±18°C and ±30°C (±65°F – 86°F). Those number are just very general guidelines.  To be more specific, the night temperature should never go below 12°C (54°F). This will cause some physiological disorders depending on how long the temperature stays below the minimum and how many times it occurs during the development stage of the fruit. So there is a qualitative aspect that comes into play when evaluating the effects of temperature. It is not just a one dimensional number that can be applied.
The average determinate tomato plant needs more than 3.5 months to grow and produce a profitable yield.  High humidity and high temperatures will result in fast vegetative growth and diseases lowering yields. On the other hand, windy warm dry climates causes blossoms to drop with dramatic decline in yields.

The same principle applies to the maximum temperatures that tomatoes can tolerate. In general the average maximum should not go over 32°C (±90°F), but that is really just important early in the morning when pollination comes into play.  High temperatures result in sticky pollination which reduces fruit set.  It is seldom that these temperatures are found early in the morning, and if it does, select a variety that is heat tolerant.

The tomato plant is not daylight length sensitive. Compare it to onions where the length of day and its change plays a role in bolting (Same with lettuce varieties especially if the temperature is above 30°C (86°F).  It must be said that a tomato will produce better quality and higher number of fruit per cluster if the days are longer and average day temperature are higher.  So a northern facing slope in southern hemisphere areas are more suitable than southern facing slopes. Northern facing slopes also provide a better angle of light penetrating the leaf canopy which provides more efficient photosynthesis thus higher yields. You can expect better tasting tomatoes with higher light intensities since ascorbic acid content is increased. You can have the most expensive fertilizer program, only light intensity will influence the acidity or ascorbic content of the fruit. The light intensity also has an effect on the colour of tomatoes.  High light intensity increases the carotene concentration of the fruit. That is why most common open field determinate varieties will have a better colour grown outside than under protected cultivation (i.e. greenhouses or shade cloth).

If you need to grow tomatoes in the winter you will have to grow them in protected environments such as plastic tunnels. In extreme cases such as central Europe and Northern America heating is required to keep the average minimum temperature above 12°C (54°F).

The specific ideal temperature ranges for tomatoes will be discussed in more detail on later pages.
Soil requirements for tomatoes

I do not believe there is an ideal soil for tomatoes. There is just to much variation in chemical make-up that influences the texture, structure, permeability, cation exchange capacity and carbon content of the soil. So if your soil has good structure, texture and aeration you should be good to go. obviously the closer to the theoretical perfect soil the closer you will get to the maximum yield potential of the variety.  Using a very sandy soil will force a commercial grower to use a fertigation system instead of conventional irrigation and granular fertilizers.  Advanced small scale farmers can get away with foliar sprays to supplement essential micro-nutrients on poor soils (or very sandy soils for that matter).  The point is you need a well drained soil for a good yield (if climate is optimum). It is interesting that on sandy or sandy loam soils a shorter growth season is achieved than growing in heavy clay soils.  Processing tomatoes again prefer clay-loam, silt loam or loam soils as long as the climate permits a long growing season.

Soil aeration and permeability can be improved by ridging and covering the top with plastic mulches, but that is extensively used by commercial vegetable growers and the subsistence small scale growers lack the understanding, labour and resources to use this technology. The open field tomato varieties are can grow in a wide pH range but the ideal is between 5.2 – 6.8.

On small scale farming and home gardening you cannot use too much compost. It is a bit more difficult with large commercial tomato growers as the volume required is sometimes impractical to get onto the farm (although some growers use spreaders to apply guano as a rich source of nitrogen rather than improving adding compost).  Compost does not only increase nitrogen content but increases the microbial content, C:N ration and basic soil structure. It is the bacterial content of the compost that is often neglected as a source of nutrients since they convert the organic compounds into a usable source of nutrition for the tomato plant. Many other micro-nutrients are also converted into water soluble form so that the roots can absorb them. If the pH of your soil is too low, i.e. below 5 or even below 4, be very careful to apply lime on a long term basis because it is a well known fact that it will lower the soil pH. It does sound strange but I will go into this fact in more detail later.  Careful management with alternative crops, application of the right fertilizers and addition of organic material can stabilize the soil pH. Also remember, it is not impossible to grow tomatoes in a soil with pH 4. You just have to select the right varieties and fertilizer combination and accept lower yields. With that in mind, having sub-optimal growing condition, a grower must spend wisely on fertilizer, chemicals, varieties etc. to protect his bottom line.

At the end the soil is only there to house the roots and keep the plant relatively upright.  Technology has advanced so much that a large portion of nutrients can be provided artificially either through the leaves with foliar sprays or directly to the roots with fertigation.
Stages of tomato plant development

Commercial vegetable growers are very pedantic when it comes to growth stages. This is because the nutrient requirements, water requirements and chemical applications change during each stage.  The crop is also sensitive to different temperatures during each stage and that determines also when the seedlings can be planted out. The climatic conditions will determine which varieties a grower can use and when.  So knowing the micro-climate of each field is extremely important if you want to be profitable.  Table 1 provides very general average temperatures that tomato plants require. As you can see, the temperature is different at each stage.

We will discuss each stage in much more detail on other pages.

  • Each stage takes a certain amount of time to complete and that is also important to know.
  • Sowing to germination: 8-10 days
  • Sowing to transplant: 28-42 days
  • Transplant to first harvest: 30-60 days
  • Harvesting period: 90-120 days (depending on variety and climate)

Effect of too high temperatures on tomato yields

  • Fruit are soft and rot easily
  • Pollen becomes sticky so that less pollen is releases causing a reduction in yield.
  • Abnormal flower growth, especially long flowers which makes entry of pollen very difficult
  • Small fruit due to higher auxen production
  • Flower drop – reducing yields again and bottom line.
  • Effect of too low temperatures on tomato yields
  • Plants grow slow and harvesting is delayed
  • Fewer fruit per cluster
  • There is less pollen development which causes the fewer tomato fruit on a cluster
  • Blotchy colored fruit
  • hollow fruit – less gel inside the tomato fruit cavity

Harvesting field grown tomatoes

The harvesting period will depend on the variety chosen as well as local climatic conditions.  Each year will be slightly different so the grower should have at least three planting times on his farm; an early crop, middle and late crop.  This is to protect himself against variations in climate and ensure good access to markets.  The planting times can be up to two weeks apart.

It is a pity there are no hard-n-fast rules on when to harvest the fruit.  It depends on the variety, the requirements of the market, climate conditions and resources.  In general tomatoes should be harvested at the ‘breaker’ stage.  That is when the shoulder of the tomato is turning pink. After that the tomato will be ready for eating in 5 days, again depending on variety.  This is applicable if standard table tomatoes are harvested.  If you are growing long life tomatoes you can harvest them when they are overall pink or red as they have a very long shelf life.


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