Growers of truss tomatoes often overlook the fact that yields are limited by shoot density. Plant density has become somewhat irrelevant now that so many growers are using grafted plants with two shoots being taken from the cotyledons. Shoot density is a more useful term that can be used to describe both single stem (where shoot density = plant density) or multi-stem plants.
When growing tomatoes for the loose tomato market, the number and size of fruit in each truss will vary through the growing season due to the effect of light and environmental conditions and the varying vigour of the plant, so the number of trusses per shoot is not the only factor limiting yield.
Markets for truss tomatoes have a preferred size or weight with a certain number of fruit in each truss. Growers of truss tomatoes prune the flowering or newly set trusses to allow only the required number of fruit to develop on each truss. This results in all the trusses being within a narrow range of weight close to the overall average truss weight.
Modern indeterminate tomato cultivars used in greenhouses typically produce a new truss on each shoot every 9 days. The number of trusses produced by a single stem plant during it’s lifetime is thus a function of the duration of the season in days divided by 9, but the number of trusses harvested is less than this by number of days taken for the last few trusses to ripen. So, for a crop which is in the greenhouse for 50 weeks, with the first truss flowering 10 days after planting, and allowing 49 days for the last set truss to ripen, 32 trusses will be picked per single stem plant (50 weeks x 7 days -10 days – 49 days divided by 9).
If a beef variety is being grown and produces trusses of 5 fruit with an average truss weight of 600 grams, then the maximum yield from this single shoot is 19.2 kg. A single stem crop of this variety at the common planting density of 2.5 plants/m2 has a maximum yield of 48 kg/m2 of truss tomatoes. Many single stem crops will yield less than this for a variety of reasons, but no matter how well the crop is managed, 48 kg/m2 is the maximum potential yield.
Increasing shoot density
Increasing shoot density by higher density planting will not be an option for most growers since it would involve a major reconfiguration of the trickle irrigation system. Shoot density is usually increased by allowing a side-shoot or lateral to grow from the leaf axil below a flowering truss. The extra shoot is taken from one plant in each slab of rockwool or coir so that water supply is the same for every shoot. The practical consideration is that one extra shoot is taken on every 3, 4, or 5 plants.
Extra shoots may be taken several times to build up to the desired stem density. Extra shoots can only be taken whenever the crop has sufficient vigour, but typically are taken at intervals of 4 to 6 weeks or so. Increasing the shoot density results in more trusses per m2 and a higher maximum potential yield, and obviously, the earlier in the life of the crop the extra shoots are taken, then the higher the number of trusses produced.
Northern hemisphere crops are often planted at the end of November or beginning of December when solar radiation is very low, so the first extra shoots are taken early in February. Solar radiation early in February in Holland or the UK is still much less than 5 MJ/m2/day. Autumn plantings are also popular in New Zealand and in Australia, but in most districts (not in the south of the South Island,NZ) winter radiation is typically 8MJ/m2/day and by August, radiation is in the 9 to 15 MJ/m2/day range. Thus, we should be able to take the first extra shoot at an earlier stage of growth than can northern hemisphere growers, but no tomato research has been done in Australia or NZ to prove this point, and many growers may be following the Dutch recommendations.
Some smaller tomato varieties, such as cocktail and cherry tomatoes, are also marketed on the truss, but truss weight is quite low and so high stem densities are essential if worthwhile yields are to be obtained from these varieties.
Growers may like to play with the simple spread sheet model (Table 1) to see the theoretical effect of timing extra shoots and shoot density on maximum yield. Setting yield targets for tomato crops before the season begins is usually based on previous years experience. Using aids like this spreadsheet could provide a better basis for setting targets, and could also be used during the growing season to revise targets as and when the extra shoots are taken. The spreadsheet model has no upper limit to density that can be postulated and, hence, can calculate impossibly high yields. I am not aware of any research giving clues as to the maximum shoot density.
The spreadsheet model is intended to apply to high technology crops grown with good light and optimum temperatures, humidity and CO2 concentrations. Nevertheless, the principle of shoot density effect on yield applies to all tomato crops.
Table 1 shows an example of the spreadsheet calculations for a crop of ‘Endeavour’ planted in week 28 at a density of 2.5 plants/m2, and with four plants per rockwool slab. The first truss flowers in week 30, and the first increase in shoot density is made by retaining the side-shoot in the axil of the leaf below the first truss. The first truss on this extra shoot flowers in week 32. A second extra shoot is taken and flowers in week 36 to reach the final shoot density of 3.75 shoots/m2. The result is that two trusses are picked at 2.5 shoots/m2, three trusses picked at 3.75 shoots/m2 and 29 trusses picked at 4.5 shoots/m2, with a potential maximum yield of 72.9 kg/m2.
Shoot density and canopy density
Increasing shoot density greatly increases the number of leaves per m2. A typical shoot, deleafed up to the ripening truss will have 15 expanded leaves, so that increasing the shoot density from 2.5/m2 to 5/m2 will increase the number of leaves from 37.5/m2 to 75/m2. The number of leaves is not physiologically important as they could be large or small and could form either an open canopy or dense, almost thatched canopy. An open canopy is desirable for a number of reasons, including better light penetration through the whole depth of the canopy, and for allowing easier air movement through the canopy and better spray penetration.
The total leaf area relative to the ground area is known as the leaf area index (m2 leaf area/m2 ground area), and the consensus from research work is that there is little if any yield increase from leaf area indexes great than 4. The canopy needs to be managed to keep it open and to avoid leaf area indexes greater than 4.
Leaf size itself is conditioned by the balance between assimilate supply and fruit (or sink) load. This is particularly noticeable in young plants before much fruit load has accumulated; their leaves are very large. This balance can also be modified by leaf picking; that is, removing a leaf not far below the head, resulting in a stronger sink demand on the adjacent two leaves, which then remain smaller than if they had been subject to a lower sink demand. Light penetration down through the canopy is important so that even the lowest leaves can contribute to assimilate production. This may require the removal of some mid-stem leaves, especially mid-stem leaves growing in the space between vee-trained rows.
Labour use for plant training is directly proportional to stem density, but conversely the higher yields from increased stem density result in more efficient use of labour for picking, since the picking rate (kg/man hour) increases with the increase in yield.
About the author
Dr John White is a horticultural consultant and author. John
provides specialist extension and research services to the
greenhouse industry in Australia and New Zealand.
Email: firstname.lastname@example.org Website: www.ghvi.co.nz Ω
PH&G May 2016 / Issue 167