Phase 2: Pricking out seedlings and grafted plants

Once the total process of sowing (germinating, grafting) is over, the plants are pricked out in the block. The block gives the plants more volume again to be able to keep growing undisturbed. The start of this phase is the initial wetting/saturating of the blocks with water and fertilisers. While the method is explained for a wetting line, the same principle applies when using other systems such as a ‘wetting cart’ or ‘ebb/flow in combination with a hose.’

For the sake of uniformity of the plant material, it is important that the blocks all be equal in terms of water content, EC, and pH. This is because the plants that are pricked out in the block must have all their needs met. The start in the block must be flawless for the plants to be able to continue to root. This is why wetting the blocks is so essential.

The speed of the conveyor also plays a role here. Try to maintain a speed of 2 to 3cm per second. Excessive speed will cause the blocks to pass underneath the spray bars too quickly and get too little time to properly absorb the moisture after each spray bar.

Additionally, the water temperature must be above 16 degrees; the optimum temperature is 21 degrees. Low water temperature leads to fibres being less effective in absorbing the moisture.

You could use a ‘Grodan water volume tray’ to check the amount of water supplied. Such a tray will let you measure how much water is supplied. The basic rule is that this should be between 1.8 and 2.5 times the volume capacity of a block.

It is important to weigh the blocks regularly and randomly after initial wetting. Make sure to subsequently also make a note of this on the prickingout list. If the blocks weigh less than the lower limit, this is a sign that the blocks are not sufficiently wet. It is important to resolve this immediately. Is there an irregular supply of water? Is there sufficient water pressure and is the flow constant? If the blocks are not properly wet, this will lead to stunted growth and heterogeneity. It is, therefore, important to regularly check and safeguard this process.

The weights of the most used block types after initial wetting:

Key points:

• Supply water in repetitions using spray bars (4 of 5 of them) to saturate the rockwool blocks;
• Ensure an adequate distance between the bars (50-60cm) to give water time to sink down through the blocks;
• The conveyor below the spray bars must be porous to allow the water to seep through the blocks;
• The conveyor must not move too fast, to ensure sufficient water is supplied and the blocks get a chance to properly absorb the moisture;
• Water temperature must not be too low;
• Performing regular scheduled checks and writing down weights is an important requirement in safeguarding the wetting 

During block wetting, different EC levels are used for different crops.

The following guidelines apply:

The pH used when wetting rockwool blocks should be between 5.5 and 6.

Grafted plants (tomato and aubergine) are also pricked out upright, so that the rootstock and not the topstock roots in the block. If the latter were to happen, the grafting would be undone, and the graft would be unusable as resistance against root disease. Cucumbers are sown directly in the block, so there is no need for pricking out. With peppers, you have the largest variation of prickingout techniques to choose from

• Standard: upright
• Generative steering: tilted 90°
• Very generative steering: tilted 180°

After pricking-out, the blocks are placed on the floor or table.
Place the grooves of the block on ebb/flow floors in the direction of the drainage channel. This way, the water drains away well after watering and the block is best protected against “wet feet”. This makes the block optimal to process with a spreader, which reduces the chance of breaking roots during planting.

On tables, the position of the grooves is only important if the surface is completely flat. The groove must then be in the direction of the drainage. However, the surface is often provided with grooves and the water will drain away under the block via these grooves. On tables, you must take into account the strongest side of the block with a view to planting.