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How to propagate seedlings

Seeds are relatively quick to plant compared to cuttings.

Collecting seeds: As with cuttings, seeds should be collected from a plant that is healthy and has favourable characteristics such as a good yield and visual appeal. Seeds should be collected when mature. Seeds that are immature will not germinate when planted. Immature seeds are soft and generally paler in color compared with mature seeds of the same plant species.

Seed storage: It is important to realize that a seed is a living organism. To suppress germination but maintain the seed in a dormant state during storage, a cool, dark and dry environment is needed. A screw topped glass jar stored in a refrigerator is ideal.

Procedure for producing seedlings

Medium: The choice of growing medium is essential. It should provide high oxygen and water levels and good drainage.

Disease: Seeds should be sterilized prior to planting. Treatment methods include:

- Soaking seeds in hot water (50OC ± 1OC / 122OF ± 1.8OF) for ~25 minutes. With this method, note the seed has a low tolerance to temperature error.

- Soaking in 1% chlorine* solution for 5-10 minutes. Unlike the heat treatment method mentioned above, chlorine only sterilizes the external surface of the seed.

Sterilization of hardware and media before and during the propagation process is paramount.

*1 part household (50g/L) chlorine bleach + 4 parts water.

Step 1. Thoroughly wash and sterilize all hardware and areas that are likely to come in contact with the seedlings and possibly cause disease.

Step 2. To increase the success rate of seedlings, use a ‘heat mat’ and ‘propagation lid’ (Fig 17.4). The vents on the propagation lid need to be closed. Maintain root and air temperature at 20-25OC (68-77OF) and relative humidity at ~80%. Note that cool conditions delay the germination of most seeds. This extends the length of time during which they are susceptible to fungal attack.

Step 3. Pre-soak the growing medium (e.g. Rockwool) by immersing or drenching with a pH buffered seedling nutrient solution. This will help remove any excess ‘alkalinity’ that is often present in the material and ensure the medium is bedded down. Allow excess nutrient to drain. If using Rockwool, gently squeeze to remove the excess nutrient.

Step 4.

Option 1. Pre-germinating the seeds prior to planting them in the medium can be beneficial because it can help identify the better quality seedlings. To do this, place the seeds between moist tissue paper on a plate. Cover this with an up-turned plate to keep the seeds in the dark. Check every few days, ensuring that the tissue does not become dry – sprinkle with water as necessary. Once the root or “radical” becomes exposed, place the seed in the medium in an upright position with the root pointing downwards. Locate ~2 to 5mm (1/8 inch) below the surface.

Option 2. Sow the seeds at a depth equal to 2-3 times their diameter. Cover the seeds with medium and gently press down.

Step 5. Immediately after planting, lightly re-water using water or bloom nutrient solution at EC ~0.8mS (typically about one-third the normal strength). Continue to water the medium as required, typically every 2 or 3 days. Ensure to maintain root and air temperature at 20-25OC (68-77OF) and relative humidity at ~80%. Diligently remove any dead leaves or seedlings - these are an ideal host for fungi.

Note: Some plant varieties or media may require little or no nutrient until the first few 'true' leaves appear (Fig 17.7b). If the success rate is poor, try feeding with plain water.

Step 6. Remove the ‘propagation lid’ once the first shoot appears (Fig 17.7a). This will help prevent fungal diseases.

Step 7. Light is not required during the actual germination process. Once the first shoot (‘plumule’) appears however, the seedlings need good light to begin photosynthesizing. This prevents the plumule from becoming spindly or ‘etiolating’. Use low intensity lighting for the first few weeks of growth. “Cool white” fluorescent lights are preferable. Position these ~10cm (4 inches) above the plants.

If seedlings are being grown outdoors, position them in a partly shaded location. Too much light can stress seedlings in the early stages.

Step 8. Gradually expose the seedlings to their proposed environment. Depending on the plant variety, this may take only a few days, or many months. Begin to expose the seedlings to increased light intensity and nutrient strength (see Table 11.30). Ensure these changes are gradual as a sudden change might kill them.

Step 9. Healthy seedlings can grow quickly, therefore it is essential to transplant them into a bigger system or container that provides adequate room for further root and shoot growth. Do this only after a minimum of 2 ‘true leaves’ have formed (Fig 17.7b).

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Related topics

"Plant propagation":

Cuttings (clones) | Seedlings | Mother (Donor) Plants

Transplanting cuttings & seedlings | Media (substrates) for cuttings & seedlings