HortFACT - Growing Greenhouse Capsicums
Capsicum plants are slow-growing, and it may take over five months from sowing to first harvest. They need a high temperature to develop at a reasonable rate. Fruit set occurs in periods and fruit are harvested in flushes.
They can be grown perfectly in soil, growing media or NFT, but they are sensitive to salt and require good water. Capsicum is prone to disorders like blossom-end rot and cracking, as well as to some pests and diseases.
The price per kg is usually high, but the production rather low: overseas top productions are in the order of 25 kg/m2 per annum for red capsicum and 30 kg/m2 for green capsicum, compared to over 60 kg/m2 for tomatoes.
Very hygienic working practices are needed to avoid spread of virus. It is important that all capsicums produced are of a very high quality standard. This requires a high level of knowledge, commitment and investment of the grower.
This article aims at outlining the basics of capsicum crop management for an almost year-round growing starting in the winter. Plant Health of Greenhouse Capsicums deals with plant health.
There are many modern varieties (cultivars) of greenhouse capsicum available. Main characteristics are the fruit shape and fruit colour (red, yellow, orange or other, while green capsicums are usually unripe-red capsicums); resistances (most modern varieties are tolerant for one or more virus types); production, fruit quality and vigour. Some varieties are very vigorous and plants can become as tall as 3.5 m in one year. Regularly new varieties (cultivars) are developed and it is worthwhile comparing a few plants of a new variety against the standard one.
Always use fresh, high-quality virus-free seeds. The seed count is 100-140 per gram, and one gram of seed will generally raise about 80-100 plants. For soil-grown crops sometimes broadcast sowing is applied with 1-2 gram per m2. Normally sowing is done in trays with 10 gram per m2 for scattered sowing, or 40 gram per m2 for space-sowing.
Sowing individual seeds in pots requires much more space, but the advantage is that no pricking out is needed. For rockwool, seedlings are sown either in propagation cubes or in rockwool flocks and later transplanted to cubes. Seed trays are filled with a high-quality soilless mix, peat, vermiculite, perlite or rockwool flocks. A complete fertiliser must be given, or in rockwool a normal nutrient solution is given but with a CF of 20 (EC of 2). The seeds must be covered with a thin layer (6 mm) of fine-grade medium, and watered with a fine spray. The soil or the trays must be covered with glass or plastic to avoid drying out.
At 28°C emergence will occur about a week earlier than at 21°C. The optimal germination temperature is 24-25°C. The young seedlings require good light conditions. Directly after germination the covering material is removed and the temperature lowered to 23°C, although 18 to 25°C is possible. The seedlings can be pricked out when the cotyledons are fully expanded (after 12 days at high temperature, or after 18 days at low temperature).
Capsicum growth is very much dependent on temperature. Particularly the root temperature must be sufficiently high (20-22°C). So for propagation under cold conditions the use of benches or bottom heat is beneficial. Capsicum transplants require a minimum day temperature of 21 - 23°C, and two degrees higher during sunshine, and a night temperature of 20°C.
Humidity must be high, but overhead spraying is not advised as it increases the risk of Botrytis. CO2 enrichment (to 700 ppm) will promote growth of the seedlings from the stage where true leaves are expanded. Good light conditions are essential. Artificial lighting supports plant propagation, but whether this is economical depends on natural light levels, prices etc.
In growing media, nutrients should be available as soon as the seedlings start growing. In peat, liquid-feet should start four to six weeks after pricking out. The young seedlings can be placed on a small area, but once leaves begin to touch they must get more space (18 plants/m2). The plants are often supported by two small sticks without an elastic band.
Plants are ready for planting when the first flower bud is visible. They have then reached a fresh weight of 35 - 45 gram. This will be five weeks after sowing in summer, or up to eight weeks under unfavourable conditions. In good planting conditions (warm peat) it is possible to plant relatively larger plants. If the conditions in the greenhouse are not optimal (low root temperature) it is better to plant smaller plants. The main root of the capsicum plant should be able to grow out freely without being obstructed by the bottom of a plastic pot.
The plant density in the greenhouse depends on the training system to be applied later (single-row, V-system, etc.). However, it is stem density rather than plant density that matters. Optimal stem density is around seven or eight stems per m2 in summer (may be higher in a new glasshouse), and around six stem per m2 in unfavourable light conditions. It is most common to have two main stems per plant, for which 3 - 4 plants/m2 should be planted.
The optimal temperature after planting is 22/21°C (day/night) on the first days, and 23/22°C later. Air humidity is best around 80 % or 4 g/m3 deficit. The root zone temperature is very important and should be around 22°C. Lower root temperature increases the risk of Pythium, while too high root temperature will cause too much plant growth and insufficient flower production.
Good soil structure and moisture content are important, and high salt or high nutrient levels are unfavourable. The optimal CF is 30, or up to 40 when transpiration is low e.g. in dull weather. The young plants require a good calcium and nitrate supply. The roots will establish within a week, and preparations for plant training can start.
The most common way of growing capsicum is with two main shoots, as this is how most plants develop by themselves. If three or four main shoots are required, the third and fourth must normally be taken from the second branching. The advantage of three main shoots is that costs for plant material is lower, while production is about equal for two or three stems per plant, at the same stem density. The difficulty is to get three equal shoots, and it is very important that the sideshoots have equal vigour. The four stems training system requires a vigorous variety.
We describe the training for plants with two stems. After the first division, two shoots develop which will form the two stems. These shoots must be trained up strings to the crop wire. Either the stems are loosely (!) wound around the strings, or better still, the stems are clipped to the strings using rings.
The plant will continue producing a terminal flower and two side shoots at every internode (Figure 1). Of every two new side shoots, one is retained to continue the stem, and the other is removed or just pruned, leaving one or a few leaves. Three leaves are left on the side shoot when the plant needs extra leaf area, either for photosynthesis or for shading the fruit. In older plants, the pruned side shoots will carry fruit.
Training and pruning should be done every three weeks, or even every two weeks in periods of fast growth. In summer it is better not to prune leaves in the upper 15 cm of the plant in order to provide shading to the fruit. At the end when the last fruit are set the plants can be topped to avoid stem breaking and to improve fruit size.
A young capsicum plant starts flowering about two to six weeks after planting, when it has 7 to 13 leaves. The first flower, or crown flower, either aborts or must be removed. Later flowers may develop into fruit. The temperature during early flower development affects the size and the shape of the later capsicum fruit. The lower the temperature during flower development, the shorter the fruit (length compared to the diameter). A flower grown under very low temperature (below 10oC at night) would produce a small flattened fruit. A lower temperature during flowering results in less four-loculed fruit, and more three-loculed fruit, or even two-loculed fruit, which is not desired. Also, flowers developed at a night temperature below 18°C usually produce a fruit with a ‘tail’ (elongated, pointing blossom-end). In conclusion, the optimal temperature during flowering is 20 - 21oC on average both day and night.
Capsicum can be self-pollinating, but there is a high degree of cross-pollination because honey bees, thrips and other insects transfer pollen from blossom to blossom. Pollination is not markedly improved by using an ‘electric bee’ nor by spraying plant hormones, but pollination is clearly better when honey bees or bumble bees fly in the greenhouse. Bees increase the number of seeds in capsicum fruit, and such fruit attract more assimilates, and therefore grow faster or bigger. Bees make that less fruit are deformed, and that fruit set and first harvest are better and a bit earlier.
In a young crop, fruit set should only be allowed when all conditions are good. Until then, all flowers and young fruit should be removed at an early stage. The main condition is that sufficient leaf area must have developed. Plants with three or four axils above the branching are big enough.
Secondly, light conditions must be good so that plenty of assimilates (sugars) can be produced at all times. Larger plants require a higher minimum light level, as they need more energy for their own maintenance. Thirdly, the flowers must be strong and in optimal condition, and best if they are bending downwards. Reducing the (night) temperature will improve the flower condition so that fruit set can take place after that.
Fruit set will occur by itself when the light conditions are good. However, when fruit set is difficult there are several methods to overcome this. The most practical way is by dropping the night temperature to 18°C. The day temperature can stay at 20-22°C, but if fruit set needs further help, the day temperature can also be dropped to as low as 18 oC. It is good if the root temperature can be dropped to 18°C as it will give more flowers. Very low night temperature (10°C) ensures a high percentage of fruit set, but induces fruit with a few seeds only, which will abort or remain small. About 18°C night temperature (and root temperature) is optimal for fruit set.
Other ways to help fruit set are: increasing the CO2 concentration (to 700-1000 ppm), and also increasing the CF (to 40). Day temperature and light are not so important for pollination itself, but high light and low day temperature are beneficial for setting and first growth of the new fruit. Fruit set takes about a week, and is normally completed in five to eight weeks after planting.
For setting of the second flush of capsicum fruit, dropping the temperature is not advised, as it will slow down the ripening of the first flush. Generally, the later fruit will set by themselves after harvest of the older fruit. Harvesting of the first flush must be done with some consideration as it significantly influences plant growth and later fruit set.
In an old crop, fruit set can be difficult especially when the light levels go down. In an old plant stand, it is important to allow for a long fruit growth period (6 - 9 weeks for green fruit, 9 - 12 weeks for red) and to get the last flush set as soon as possible. This can best be done by harvesting the plants empty, even harvesting green, and then setting night temperature very low (even as low as 14°C) until fruit set is completed. Raising the CO2 level will help too. Fruit ripening can be sped up after the harvestable-green stage by higher temperature (24°C average).
Each plant can raise a limited amount of fruit, either limited in number or in weight. Too much fruit load on a plant may cause decaying of roots, and as a consequence Pythium may attack. The number of fruit a plant can raise depends on light, temperature and plant size. Young plants can support only 12 fruit per m2 under poor conditions, and over 24 fruit per m2 under good light conditions (this is four to eight fruit per plant, or two to four per stem). An older crop can have over 40 fruits per m2 at a time. Mature plants control fruit load themselves by fruit set and fruit thinning. In young plants, however, fruit load must be controlled by the grower, in order to achieve a reasonable fruit weight (e.g. minimal 150 gram per fruit).
The best method to correct the number of fruit is by removing excess. This improves fruit quality (as inferior fruit are taken out) and fruit size (as remaining fruit experience less competition). The earlier the fruit thinning is done, the more the other plant parts (fruit, shoots and roots) will benefit. However, if too few fruit are left on the plant, these are more at risk for fruit cracking.
Secondly, fruit load can be corrected by raising the temperature just after fruit set. This reduces the amount of assimilates available, which will cause abortion of some new fruit. However, this method does not have the positive effects on fruit quality. Thirdly, plant load is affected by harvesting.
About 5 - 9 weeks after fruit set, fruit will reach their full size and get firm, while still green (the ‘harvestable-green’ stage). Further ripening on the plant until fully-coloured takes another 2 - 4 weeks. The total period from blossom to full-colour ripe fruit is 7 - 12 weeks. The huge variation in fruit growth duration is due to many factors: season (light), temperature, fruit load, condition of the plant, plant management, cultivar, etc.
Fruit growth can be hastened by high temperature (around 24°C). Generally, one degree higher average temperature reduces the fruit growth period with two to three days, but high temperature also reduces the average fruit weight. High temperature is not recommended under poor light conditions. Too high temperature (above 28°C) stimulates blossom-end rot, and too low temperature (below 17°C) at early fruit growth may cause a ‘tail’ on the fruit.
Capsicum fruit can be harvested when they are harvestable-green. Green fruit are incapable of ripening after removal from the plant. When harvested a week after the harvestable-green stage the fruit will turn gradually into colour, which is not desired. The right stage for red fruit is when they have reached full colour and are still firm. Overripe fruit are soft, and have a low keeping quality.
Capsicum fruit must be harvested with a very sharp, small knife, to get a smooth stem-end appearance, and so to minimise damage to other fruit. Also, a small wound on the plant reduces the risk of Botrytis infection. If plants are suspected of having a virus infection, the harvest knife should be dipped in low-fat milk with 3.5 percent protein to capture the virus and avoid virus spreading to other plants.
For delivering good quality fruit, it is best to start harvesting early in the morning and to finish before the hottest hours of the day. It is most important that harvesting, handling and packaging are done with the greatest care. Capsicum fruit are very prone to handling damage, which usually becomes visible after storage.
Fruit will mature in flushes, certainly in the beginning of the production. In peak periods frequent harvesting is needed: coloured fruit once or twice a week, and green fruit once per fortnight. After a harvest peak it is still good to make a quick harvest round e.g. once a fortnight to pick the odd late or early fruit.
Harvesting is ‘unloading’ the plant, and allowing energy to go into new growth. New shoots will develop and produce a new flush of flowers and young fruit. Harvesting green fruit will result in an earlier unloading, and thus earlier setting of new fruit. Green harvesting can be recommended for the first flush when the plants are still small. If only some fruit are harvested green, the other fruit are more at risk of cracking, so for that reason it is better to harvest all first fruit green at once.
On the other hand, when all fruits from one stem are harvested at the same time, the sudden change in fruit load may cause a too vigorous growth of that stem. It will produce flowers that are too coarse, which will develop in low quality fruit. So the last ripe fruit should be harvested after some new fruit have set on that stem. The effect of harvesting on shoot growth can be used to achieve equal length of the stems on one plant.
Young plants generally require a high average temperature to stimulate the appearance of new leaves and development of leaf area. A typical minimum temperature for young capsicum plants would be around 22°C (21- 23) (average for day and night). This can be achieved with for instance 25/19°C (day/night), or with 22/22°C. The latter regime will give more compact plants. If the young plants are planted in summer under high light conditions, the average temperature can be set one or two degrees higher. Also during flowering the night temperature and the average temperature must be relatively high (minimum 20 - 21°C) for a good flower quality.
For good pollination and for getting fruit with many seeds a lower night temperature (18°C) is optimal. Day temperature can remain at 20-22°C, or can also drop. Directly after fruit set a moderate average temperature (20°C) is set for growth of young fruit, or a higher temperature (22°C or more) can be chosen to stimulate abortion of excessive fruit.
During early fruit growth the mean temperature (24 hour average) can be chosen between 20 and 23°C depending on the conditions. An average below 19°C may give rise to ‘buttons’ and ‘tails’. A higher average temperature (23°C) hastens fruit growth, but reduces the ultimate fruit size. To achieve 20°C on average, often the day temperature is set around 23°C, and the temperature in the first half of the night is set low, e.g. 16°C, because a low night temperature gives bigger fruit and better keeping quality. The maximum temperature should ideally not come beyond 28°C.
The ideal method for heating would be by heating pipes, where the lower pipes heat the roots and are also used as rails for the trolleys. The higher pipes can be at the height of the fruit. In cold winter conditions it can be economic to use a thermal screen.