seedlings; pesticides

Hydroponic farming: How to produce vegetable seedlings

Vegetables are essential for good health, and to ensure you get the best, you have to follow some sound principles for growing them – especially if you decide on hydroponics.

  • Vegetable crops may be seeded directly in the field or planted in seedling beds and transplanted later.
  • Crops that are grown in a hydroponics system need to use seedlings produced in a clean, disease-free environment.
  • This is especially true for tomatoes, peppers, lettuce and other crops that can be transplanted easily.
  • Planting diseased plants usually results in major problems and financial loss.
  • One diseased seedling can infect an entire closed hydroponics system.

If, as a beginner farmer, you don’t have the infrastructure to raise your own seedlings, you should buy from a reputable nursery. We recommend, therefore, that you visit the seedling nursery before placing an order and also during production.

You can recognise a reputable seedling producer by:

  • Reputation. What do other producers think of the nursery?
  • The personnel’s experience.
  • Sanitation and vigilant pest and disease management.
  • Specified quality standards. Does the nursery detail the height, age, etc of seedlings?


1. Choosing a cultivar

  • Choosing the right cultivar is very important and depends on the crop, area and target market.

2. Planting date – planning seedling production

  • This depends on the cultivar and crop.
  • If the wrong cultivar is chosen for an area, the planting date on its own cannot make it a successful crop.
  • If a summer tomato cultivar is sown in winter, for example, it will not perform well.

It is very important to have some knowledge about the cultivars and there are only 2 ways to ensure this: to use a trustworthy nurseryman, or have a good relationship with a seed company that knows your area and targeted market.

3. Ordering seed

  • The planting date indicates when to sow and seeds must be ordered in time.
  • This should be done at least 4 weeks before planting so that seedlings will be strong and established and ready for transplanting.
  • Market requirements and price forecasts determine sowing and transplanting times.
  • If sowing takes place too late, transplanting will be too late and the marketing opportunity will be lost.

4. Equipping the nursery

  • If a grower wants to produce seedlings for his own use, the nursery can supply to suit his needs.
  • If seedlings are to be produced on a commercial scale, the situation (scale) changes.
  • The most important elements are to ensure that seedlings receive enough sunlight to prevent elongation and weakness.
  • Irrigation and fertilisation are also important.
  • The seedlings should not be stressed while developing; and they should get enough fertiliser to ensure they aren’t weak and prone to pests and diseases.

5. Preparing trays

  • Seedling trays are expensive, but they can be cleaned and used again.
  • Sterilise used seed trays with Sodium hypochloride or Sporkill™ to destroy fungi and bacteria before using them again.

A word of caution: prolonged use results in the planting sacs becoming porous, and if you use them, the roots grow into the small holes, and the root system is damaged when the seedlings are removed.

  • Seedling tray cavities shouldn’t be too big, as you will then use more growing medium than necessary to fill them.
  • They should, however, be big enough for the seedlings’ root system to develop without major restriction for at least 3 to 4 weeks.
  • The medium should be well aerated, and have a good water-holding capacity.
  • The pH of the medium has to be between 5.5 and 6.5.
  • Peat, bark or vermiculite mixes are generally used.

A too-fine medium often results in poor drainage; a green layer of algae on the surface usually means poor drainage or too much water. Once algae are established, diseases such as damping off can become a problem. There will be competition for nutrients between seedlings and algae and the plants will become yellowish and will not develop well.

If the medium is too coarse, the water will easily run through and the medium will dry out too quickly. Root growth is also poor. In order to ensure that aeration and water drainage are good, and to prevent the spread of disease, you should always make sure that seedling trays are not in direct contact with the soil or the floor of the greenhouse. Place trays on benches that have mesh or wire tops.

6. Sowing the seed

  • You can sow seed by hand or machine.
  • The planting depth of the various crops differs; some seeds need light for germination, others don’t.
  • If conditions are favourable, seedling trays can be placed directly in a tunnel.
  • The medium must be moist when it’s rubbed into the tray.
  • Make sure each hole is filled properly otherwise the seedling won’t have enough medium in which to develop a good root system.
  • Make hollows that are three times the diameter of the seed.
  • Place one seed in each hollow and cover it with a thin layer of vermiculite or other material that won’t form a coarse crust on the seed.
  • Use a very fine nozzle to irrigate so you can be sure the seeds and medium do not get washed out.
  • The medium should never dry out or be too wet.

7. Germination

  • If good-quality seed is used, the germination rate should be 100%.
  • Check that there’s only 1 seed per hole and that the tray is full.
  • Make sure the seedlings are disease-free and all at the same stage of growth, otherwise the crop will not be ready at the required time.
  • The kind of crop determines how long germination will take, but it will be at least 5 to 10 days. (Both roots and leaves have to develop.)
  • Fertigate the seedlings after the first true leaf emerges (6 to 8 days after germination).
  • You can use a seedling fertiliser such as Multifeed, but make sure you give the plants the right dosage.
  • Don’t use too strong a solution – it could burn the roots and the seedlings may not recover.
  • To protect small seedlings against sunlight, hail, birds, and so on, use the appropriate covering, such as shade net or hail net.
  • And remember that the infrastructure must let in enough light, and also prevent too-high temperatures.

8. Hardening off

  • It is important to harden off the seedlings before they are transplanted.
  • This means exposing them to the environment in which they will grow.
  • If they were produced in a mist bed in a greenhouse, harden them off gradually.

9. Transplanting

  • Before you remove the seedlings from the tray, make sure they have not been damaged, and that they have been properly loosened from the tray.
  • Don’t pull the seedlings out of the trays – you could damage the roots.
  • Prepare the planting bags, or the gravel, before the seedlings are ready to be transplanted, so that you don’t waste time after they’ve been hardened off.
  • Make sure the drippers and water flow are installed before transplanting; if the seedlings are transplanted before the irrigation is in place, they will stress, weaken and not produce a good yield.

The various crops are transplanted at different times:
Tomatoes: 5 to 6 weeks
Peppers: 6 to 7 weeks
Cucumbers: 4 to 5 weeks
Lettuce: 3 to 4 weeks.

Also read: Hydroponic farming: The basics – popular systems

  • This article was written by Erika van den Heever and first appeared in Farming SA.

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