How I use grow blocks

How I use grow blocks

The best method of germinating seeds is using a system that is the easiest to use for you and one that yields the highest germination rates that are acceptable to you. I know that certain methods used by commercial operators are higher yielding in a lot of circumstances, but then an amateur is happy by getting an acceptable yield to satisfy his immediate planting requirements.

As amateur gardeners we need a variety of seedlings at the start of the growing season whereas the commercial operator will produce vast numbers of identical cultivars and his methods will differ from ours in the approach, not saying that we cannot copy good techniques and apply it in our own cases whenever possible.


Seeds vary in size and therefore need different treatments when preparing for germination and I use different blocks or media to start the process. Let me put the seeds into 4 categories as follows:

Large                                   Medium                             Small                                      Fine

Beans, Peas…                   Cucumber                           Tomatoes, Capsicum          Oregano, Basil

Direct into Perlite             75mm grow block            Oasis blocks                          Rockwool blocks

Grow Blocks

Let us first look at the most common blocks, Oasis, and Rockwool, and how they behave differently when in use. In the first instance we have an Oasis block in a cut away section, full Oasis block and finally a Rockwool block and the extent of capillary action in wetting the blocks in 24 hours standing in water. The Rockwool showed slightly higher capillary action then the Oasis but none of them showed wetting of sufficient height to prove effective (Photo 1)


Photo 1 above & Photo 2 below

If we soak the blocks and let them drain, then we get a different perspective of the retained water after 24 hrs as is shown in (Photo 2) where the moisture level is about where the seed should be located. With the Rockwool block it is difficult to judge where the correct moisture level is for the seed and many time germination fails because the block is to wet.


Photo 3                                                                            Photo 4

In Photo 3 we show how to deposit a Capsicum seed into the open section Oasis Block using a pair of tweezers to place the seed or alternatively you can drop the seed in the hole and then use the blunt end of a wooden skewer to push the seed into place. Photo 4


It is not important what shape or size the container is, it all depends on a number of conditions:

  1. Water tight
  2. Transparent cover
  3. Has water holding padding
  4. And a weed mat layer to stop root ingress into the pad


See Photo 5 to illustrate the layers.


In photo 6 we show placing a capsicum seed into the grow block which has been fully soaked, has a residual layer of water in the pad and is held at about 25⁰C. Once the baby leaves have emerged we change the solution in the reservoir from water too nutrient and apply light for 14-16 hrs. a day. When the conditions are right you should see condensation on the cover   indicating that the humidity is saturated.

Last meeting, I mentioned that I use a technique with the Oasis blocks where I put a spare seed into the side, as well as the centre of the block, to provide additional seedlings in case some fail to germinate and in this manner I can guarantee that I will always have the required number of seedlings at one time. Using tweezers, I push the seed about 5-10mm into the side of the block and most of them seem to germinate and its only selecting the best plant to grow on.

Photos 7 & 8 show a number of blocks showing both centre development as well as side growth and I have shown how the spare in the front row is removed and placed into a failed hole to make up the numbers required and the rest wasted.


Photo 7                                                                          Photo 8

When we have to use fine seeds such as oregano or basil I generally use a Rockwool block, sprinkle the wetted block with the seeds and gently tap them into the surface with a finger. It will be necessary to occasionally spray the surface with water before it dries out. It is possible to use the same technique using Oasis Blocks as is shown in Photo 9


Photo 9

For Cucumbers I use a 75mm block with a hole and fill the hole with Vermiculite and before use, you have to change the pH of the block from alkaline too acidic by soaking it in a 5.5pH solution of Phosphoric acid or as I choose, soak it in nutrient of similar Ph. value. At least in this way the plant will have the feed when it emerges and needs feeding. Using tweezers again, I insert the seed about 20-25mm into the water soaked vermiculite and wait for the results as is shown in Photo 10.

One other technique I use is to prepare a grow block which I can insert into a 50mm hole in a tray system using Peter Signal’s method of holding the plant in place with a short length of plastic pipe, is to take an Oasis block with the tap root already showing at the weed mat interphase and place it onto a suitable size grow block which will slip into the holding pipe. That grow block has its plastic sleeve raised by abut 10mm to provide horizontal stability for the Oasis grow block as is illustrated in Photos 11 and 12. The plant has to be fed with nutrient from the top until the root system takes over in the lower Rockwool block and transplanted when the roots hit the bottom.



Photo 10                                       Photo 11                                  Photo 12


And finally we show the direct sowing of large seeds in coarse Perlite Photo 13 and the end result of Photo 12 when the seedling is grown in a tray system Photo 14.


Photo 13 (Beans direct sown in perlite)                    Photo 14 (Capsicum in tray system)


Gerry, 25 August 2016

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