If you have grown peppers with other crops, you might have noticed that green peppers grow slower than most others. Even the seedlings take about 2 weeks longer before they can be transplanted. Actually, green peppers grow just as fast, but the perceived vegetative growth is much slower. Green peppers are not less productive than other plants. Pepper leaves are just thicker (especially the seedlings) than other plants, so there is more vegetative material to produce for the same leaf area. Some scientist have tried to reduce the thickness of the leaves by reducing the incident light reaching the plant, but that seemed to be counter productive since the productivity of the plant decreased. The main aim is to increase the leaf area of the seedlings -so that yields can increase later on, the trick was to use 25-50% shading during the seedling growth phase. This only worked in tropical environments with higher humidities.
So what is the best way to increase pepper growth. I hear a lot of growers saying, increase the speed please, no, what you need is increase productivity of the pepper plant. They best way to do that is by the air temperature inside the greenhouse. So you have to optimise the climate inside the greenhouse for best growth. Here is why: the rate of plant growth (it’s productivity) is significantly affected by the air temperature, it affects both rate of dry matter production (so how fast it grows and increases weight) and very importantly, partitioning of the plant material produced to where it is important, i.e. in the leaves. During the initial vegetative stages (thus before fruit development, that will be reproductive stage), the optimum temperatures are 25-27°C during the day and 18-20°C during the night. The average temperature should be between 20-22°C for optimum growth development. As soon as the day temperature goes below the night temperature, significant decrease in growth rate will be experienced, and that will have an influence on yields. Never let the temperature fall below 12°C during the night. Low night temperatures increases the leaf thickness, and that is exactly what we do not want. In scientific terms the ratio of leaf area to total plant dry weight will decrease.
So what about root growth. Just remember that root growth during the vegetative stages of the pepper plant, develops at the same rate and above ground parts. Since roots are either planted in containers or channels, they have limited potential for expansion and will not grow as much when planted in a deep soil (which can reach lengths of 3m!).