Grow three indoor plants — Part 2: Substrate selection and germination

In the first part of our series we reviewed the equipment you cannot do without for indoor cultivation. Now we will focus on the foundation of success — the right growing substrate and germination. We will advise you how to choose a suitable medium, how to work with it and how to get healthy, strong seedlings from seeds.

In the previous installment of our series we introduced the equipment you will need for indoor cultivation. We also answered why it is better to grow from seed and in a another article on our blog we went into detail on choosing seed for your herb garden. Today we will help you choose the right growing substrate and guide you through germination.

Choosing a growing substrate

Indoor cultivators are often pioneers of new horticultural techniques, which is also true for the use of various innovative growing substrates. The medium for plants does not always have to be soil, as it is in nature. It can be replaced in indoor growing by, for example, coconut fibres, mineral wool, expanded clay or another suitable material.

Each growing medium has its specifics and suits a different growing style. Soil and potting mixes have universal use and can be combined with mineral and organic fertilizers. Inert substrates (rockwool, expanded clay, perlite) are used in hydroponic systems and require precise nutrient dosing and pH control. Coconut substrates sit between soil and hydroponic media. Although they require tailored fertilizers, unlike purely hydroponic media they better resist pH fluctuations.

Which substrate is right for you mainly depends on your preferences and experience. Beginner indoor cultivators often prefer to try soil mixes first and only move to coco or hydroponics once they have mastered the basics. Remember that crop quality depends less on the substrate used and more on how you care for the plants and the healthy environment you create for them.

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Soil and potting mixes

The traditional and still most popular growing medium. Depending on the age of the plants and the growing style, you can choose from light substrates, moderately fertilised mixes to heavily pre-fertilised soil. Potting mixes are suitable for growing in plastic and fabric pots (for flood tables) and raised beds. When growing in soil you can water plants manually or use an irrigation system.

• Advantages: Natural nutrient content, good buffering capacity (resists pH swings), possibility of organic growing.
• Disadvantages: Low-quality mixes may contain pathogen or pest eggs, soil compaction, overwatering.

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Inert substrates

By inert growing substrates we mean those made from materials that do not react with nutrients and water. Most commonly this is mineral wool (rockwool), but expanded clay, vermiculite or perlite can also be used. Since they contain no nutrients, these media are suitable for hydroponics, where plants are supplied with nutrients in the form of a nutrient solution. This solution must have the appropriate pH and its preparation requires special water-soluble fertilizers that must be precisely dosed for the plants. Under certain circumstances inert media can be watered manually, however most growers use irrigation or hydroponic growing systems.

• Advantages: Sterile growing medium, high aeration, precise plant nutrition enabling efficient cultivation.
• Disadvantages: More demanding preparation of nutrient solution, low tolerance for grower errors, environmental footprint.

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Coconut substrates

Coconut substrates are made from coconut husks and combine properties of soil and inert substrates. Coconut fibres retain water and air better than soil, however they contain no nutrients by themselves. When growing in coco it is therefore necessary to use fast-acting fertilizers, although compared to inert media coco can to some extent be combined with organic inputs. Coconut substrates are suitable for growing in plastic and fabric pots (for flood tables) or raised beds. When growing in coco you can water plants manually or use an irrigation system.

• Advantages: Neutral pH, versatile use, excellent retention properties, possibility to inoculate with beneficial microbes, suitable even for beginners.
• Disadvantages: Requires use of special fertilizers and control of pH and EC, coconut fibres must not dry out completely.

Germination

Compared to sowing seeds outdoors, where success is up to nature, in indoor growing you can create ideal conditions for germination. The main factors that influence germination are:

• Temperature: Seeds germinate best at temperatures between 21-26 °C. Low temperatures slow germination and increase the risk of damping-off fungi. Germination success is also significantly reduced by temperatures above 30 °C.
• Moisture: On contact with water the germination process in seeds begins and nutrients stored in the seed are released for the seedling to use in the first days of life, before they can photosynthesise. Water for germination should be free of pathogens, so it is better to boil it or use distilled water.
• Light: Seeds do not require light to germinate, but once they appear above the surface of the substrate they will need it to orient themselves and for photosynthesis. A weak grow light of a few watts is sufficient for this.

Germination

It usually does not pay off to germinate in large growing containers filled with a lot of substrate. Seeds need stable moisture and temperature and this is easier to achieve in a small pot filled with seed-starting or coconut substrate or in seedling cubes made from various materials. Starter plugs made from organic materials (soil, peat, coco) are suitable for growing in soil and coco, but not for hydroponic systems. For those it is better to use rockwool or germinate in water.

Germination in three steps

1) Moisten the growing substrate: If you are germinating in pots, first moisten the substrate sufficiently. The soil should be soaked but not waterlogged. You can test the moisture of soil or coconut substrate by squeezing a small amount in your palm. If a few drops of water come out between your fingers, it is just right.

Seedling cubes just need to be soaked in water before use. If you care about maintaining sterile conditions during germination, you can soak the cubes in a weak solution of hydrogen peroxide (0.5 %), which will kill microorganisms.

During germination you can also use compost teas and microbial inoculants that contain beneficial microorganisms protecting seeds from fungi and supporting plant health and growth. These products cannot be combined with peroxide, which neutralises them.

2) Plant the seeds: Once the substrate is prepared you can proceed with sowing. Plant seeds to a depth of about two to three times their length. If you are growing different cultivars, be sure to label germinating seeds clearly.

3) Maintain proper moisture and temperature: Germination proceeds best at room temperature and high ambient humidity. The easiest way to achieve this is in a small plastic propagator, where you can hide the germinating seeds. Place the propagator in a room with a stable temperature and make sure direct sunlight does not shine on the seedlings.

In the following days check the substrate moisture daily and if it dries out, moisten it with water from a spray bottle. Do not pour water directly onto germinating seeds to avoid washing them out to the surface. Seeds should germinate within 7 to 14 days depending on seed age and quality. Once seeds have germinated, move them under appropriate grow lighting with a vegetative spectrum.

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In the next installment we will focus on seedling care, transplanting and the first pruning. You will also learn how to choose fertilizers for indoor cultivation and when to start feeding the plants.