Production of Summer Vegetables in Winter under Naturally Heated Plastic Tunnels (Part.2: Climatic requirements, Seedling raising & Transplantation)

, posted in Greenhouse

By Sajid Iqbal Sandhu, Green Circle of Pakistan


Plant growth and fruit production of tomatoes, cucumbers and sweet pepper is directly related to the environmental factors such as light, relative humidity, temperature, carbon dioxide, water and nutrients.

A low level (excess level can also occur) of any factor will reduce plant vigor and quality. Water and nutrients have already been discussed and can be controlled with the various tools mentioned.

Regarding light, in general under Pakistan conditions light will be hardly a problem, unless with very dirty plastic/glass covers or prolong fogy conditions. Carbon dioxide shortage may only occur when tunnels/ glasshouses are 100% sealed off from the outside, however Closed greenhouses are the remedy in this regard. Remaining are temperature and relative or atmospheric humidity. 

• Temperature:

Every kind of vegetable has its own optimum temperature for germination and its subsequent growth.

Below is mentioned the number of days required for seedling emergence at various soil temperatures.

When raising seedlings, try to reach and maintain the optimum temperature of approximately 25-30°C, because at lower temperature the seeds are only longer exposed to possible disease infections.

Especially in the area where early spring planting is being practiced, it is worthwhile to create a special seedling raising space with some heating.

Also do not forget that after transplanting, for a quick start and optimum growth a rather warm climate for those vegetables is required.

However, the experience is that in greenhouses/tunnels without additional heating, large fluctuations between day and night temperatures do occur sometimes more than 30°C between maximum day temperature and minimum night temperature. Still it should be tried to reach the mentioned temperature as close as possible. During warm hot days, ventilate as much as possible and/ or if temperatures stay continuously high, apply shading or chalk on the glass/plastic in order to reduce the radiation.

Regarding higher temperatures at night, the main solution would be the installation of a heating unit, the main solution, would be the installation of heating unit, of course double plastic layers may give some more isolation than a single plastic layer, but the differences will be minimal.

• Relative humidity:

Concerning the relative humidity, this should not be too high as it facilitates diseases, particular downy mildew and botrytis. However in the winter periods during the night, the relative humidity increases as close to 100% and for a rather long day under sunny condition, the relative humidity drops to 25-35%.


In order to obtain an uniform crop (plants equal in development and health), it is advisable to raise seedlings in pots, blocks or any other type of container. Direct sowing is still being practiced, but this should be only restricted to cucumber and only then when the soil is well prepared and with an optimum soil temperature of around 25°C to ensure a quick and even germination.

For tomatoes and sweet pepper (which do have a longer raising period) and for cucumber (under cold circumstances), raising of seedlings is preferred for the following reasons:

a) Since little space is required, it is easier to control for the grower.

b) Special adaptations can be made to control the environmental conditions to certain extant.

c) Off types or weak seedlings can be removed before transplanting.

d) Use of special germination media for a quick start.

For raising seedlings a great choice of containers are available such as soil blocks, plastic pots, jiffy pots, speedling trays and many other types.

In combination with a suitable medium for a quick germination, they all may give healthy strong seedlings.

However the final choice depends on the cost of container, including the cost of the quantity of media it will take and whether they are easy to handle. Just think of the transport of seedlings from the raising space to the place of transplanting.

Also the choice depends on what kind of plant is desired. As noticed, due to the relatively high cost of a good medium, there is a tendency to use a quite small container and for this reason it should be remembered, the smaller the container, the earlier the plants should be transplanted.

• What should be the characteristics of a good medium?

For a quick start, the medium should have a high volume of airspace, so that good aeration, a good water holding capacity, a low soluble salt level and free from pests and diseases, is ensured.

Very good results have been obtained with peat moss. But be careful, take the coarse grade, not the fine one, because this grade has the ability to hold water well in the small pores and the larger pores are filled with air and therefore a good supply of oxygen provided which is essential.

Of course other growing mediums or soil mixtures may be good as well, but when one has been using a kind of growing medium satisfactory for some time, then the grower should not change to another medium or mixture before having it tried out on small scale and also the grower should stay in touch with his fellow farmers/growers.

As had been noticed, many costly mistakes have been made by using a not suitable medium.

• Sowing:

In general the seeds should not be sown deeper than the diameter of the seeds. Deeper sowing increases the chance of rotting and uneven germination because of poor aeration.

Moisten the medium well before sowing and keep it moist until the seed has started germination. In case water has to be added, give it only with a very fine droplet.

Directly after sowing, the humidity in medium can be maintained by covering the containers or trays with plastic, paper or other material until the seedling are almost breaking through the soil surface.

Be careful when plastic is being used, because under full sunlight, heat can build up rapidly under the plastic and damage the germination. Water periodically when necessary to maintain the desired moisture into the medium. Do not over water, excess watering may result in “damping off”. May be a combination of "spot watering" of dry areas and an overall watering may be required. Also here, apply water with fine droplets.

• Transplanting:

In general early transplanting is beneficial to the later growth of the plant. Avoid situations such as overcrowding and too large plants in small containers.

Overcrowding will give thin and spindly seedling and big plants in small containers may get quickly exposed to nutrients problems.

When transplanting, place the plants carefully in the well-prepared and moist soil and press the soil firmly around the block with the roots without damaging. Add water, "water the plants", at the foot of the plant which will give a good contact between roots and soil.


ABout Author: Sajid Iqbal Sandhu is CEO of The Green Circle Pakistan (Innovative Agriculture Technologies). Besides this, he is a Tunnel Expert, Importer, Distributor, Seed Acclimitizer, Agri Consultant and Freelance writer. Email:

Copyright Hortist, 2013

More from Sajid Iqbal Sandhu

  • Urdu Book on Tunnel Farming (Part:2)
  • Urdu Book on Tunnel Farming (Part:1)
  • Black Pepper production in Pakistan (In open field & Under high tunnels)
Hortist is a sustainable source for landscape horticulture news, solutions and resources, managed by a Landscape horticulturist from Pakistan. Hortist reports on importance of this very unique niche and how it improves the landscape of this world and lives of its inhabitants.