To germinate tomato seeds, first of all, the seeds are necessary. These have to be healthy, mature and have the capacity to germinate, so they cannot be very old or have been exposed to poor conservation conditions.
Support for germination
Secondly, a porous support is necessary in which to deposit the seeds so that they can remain moist during the germination process.
The substrate for seedbeds is one of the most used supports to germinate seeds, but absorbent paper —kitchen paper— or cotton can also be used.
In case of using any of the latter, you would actually be making a pre-germination. After this, the young plants will need to be moved to a pot with substrate or to the ground, so that they can take root and extract nutrients through the roots. Paper or cotton cannot supply the necessary nutrients.
In any case, it is essential that the support is capable of retaining water in its pores so that the seeds remain moist throughout the process.
Pre-germinated on paper or cotton
For this, a piece of absorbent paper or two layers of cotton is used, or any other inert and absorbent tissue, which is inserted into a tray or container with a lid, for example a tupperware.
Step by step process
Only 4 very simple steps but that must be carried out with great delicacy.
1 Place seeds on the paper
The paper is folded in half twice, and right in the middle will be where the seeds are placed. They will remain between 4 layers of paper, two above, and two below.
Before placing the seeds on the paper, it is completely moistened, for example in a glass of water or under the tap. Then it is squeezed in the hand to drain excess water. It doesn't matter if it gets wrinkled, if it's kitchen paper it can be stretched again.
Once drained, it opens in half as if it were a book, and the tomato seeds are deposited on one of the two sides. They have to be well distributed but without getting too close to the edge. At the ends, moisture is more easily lost.
Once the seeds are arranged on the paper, it is closed and lightly squeezed by hand. This is important so that the seeds remain in contact with the paper, both above and below.
Then you have to put the paper with the seeds in something that prevents moisture from evaporating quickly. Inside a tupperware (more ecological) or wrapped in aluminum foil. If this last method is chosen, it is convenient to save the aluminum foil for future germinations, avoiding generating a residue.
3 Store in a warm environment
The container is covered and placed in a warm (16-28°C) and dark place, or with little light.
Since most of the tuppers are transparent, the light would pass through them, just like wet paper. It would get to the seeds by "making them believe" they are out of the ground, so germination could be delayed. If the container is opaque, such as aluminum foil, the requirement for darkness is not necessary as light will not be able to reach the seeds.
Where to put them inside the house? Well, for example, near a heating radiator, never on top, on an electronic device that gives off heat, such as a router. You can check the temperature of the area with a thermometer. If it is between 20 and 25ºC it will be enough.
Outside, in a greenhouse, it is also possible to germinate them, as long as the weather is sunny and the nights are not too cold.
4 Daily control
Once a day you have to review how the process is going.
Open the container and paper, check for signs of germination (radicle exit), cover the paper again and leave it in the air for about 5 minutes to ventilate. If it is not aerated, excess moisture can lead to the appearance of fungus, endangering germination.
If the conditions are right, germination will take place after 5 to 8 days. With healthy seeds kept at a temperature of about 25ºC, in 5 days they will have germinated.
Transfer to substrate
As has already been said, once the seeds have germinated, the young plants must be transferred to a substrate that can provide them with the necessary nutrients to continue their development.
5 Transfer to pots
Once it is observed that the seeds are clearly germinated (the white radicle and the greenish or yellowish cotyledons are visible), they must be carefully transferred to individual containers filled with substrate for seedlings.
The pots can be small (like a yogurt cup) or larger. The smaller they are, the sooner a transplant to a larger one will be necessary.
To transfer the seeds from the paper to the substrate, you must take them one by one very carefully so as not to damage them. A pair of tweezers is well suited for this job.
Depending on how advanced the germination is, the tomato seeds will have to be arranged in a different way.
Seeds with radicle only (white bud)
This outbreak is the first thing that appears. If the leaves (cotyledons) have not yet emerged, then it is enough to place the seeds on the substrate, with the tip of the radicle pointing downwards. If it is very short, it is not necessary to bury it, but if it is not, it will be necessary to make a small hole with the tweezers or with a pencil, to introduce the radicle, leaving the seed to remain on the surface.
Seeds with radicle and cotyledons (primordial leaves)
If you have waited too long and the cotyledons have already emerged from the seeds, then you have to do the same as the previous step, that is, make a hole, introduce the radicle in it up to the height of the seed (it may be a little buried). , and leave the stem (if any) and cotyledons out of the substrate.
In any case, you have to be very careful when you cover the root. You can push the substrate very gently with tweezers or a pencil, without pressing too hard but making sure that the radicle is in contact with the soil.
6 First watering
It is watered generously with a very fine rain spray or watering can so that the seed, and especially its root, is well attached to the substrate.
It may be that when watering, the substrate softens and the young plants (if they already had cotyledons), fall to the sides. If this happens, you have to add a little more substrate around them while they are standing upright.
7 Development conditions
Place the pots in a warm place with good lighting, but protected from direct sunlight. A veranda, window sill, or conservatory are good locations.
From this moment, it will only be necessary to maintain a constant but not excessive humidity in the substrate, and to watch if the plants grow healthy and vigorous. If little growth is observed, it is necessary to make sure that the light and temperature are sufficient, and if it is due to an infertile substrate, it can be watered with natural liquid fertilizers, such as nettle slurry or vermicompost leachate, among others.