Seedling Mixes

There are several mixes available on the market that are designed specifically for the use of seed sowing and growing seedlings. These mixes may contain sphagnum peat moss (and/or sedge peat), vermiculite, perlite and a wetting agent. This is what you would call a soil-less mix, and can be described as a lightweight, freely draining medium with good aeration and moisture retention capabilities. Wetting agents are generally added to peat moss to help with rapid water absorption. Vermiculite is a mineral similar to mica with a flat, plate-like structure that holds water, nutrients and air. Perlite is a more rounded, neutral volcanic rock particle and also contributes to the aeration of the mix, as does peat moss.

A good quality mix should have been sterilized at some point. This is important if you want to prevent any nasty moulds, diseases, bugs and other pathogens from developing. Most mixes will also contain a degree of the essential macro and micro-nutrients or elements that all plants need in order to grow, but these tend to leach out directly and disappear before too long.

Actually, until the first true leaves appear, nature has provided any germinating seedling with all the energy it needs in the form of carbohydrates, proteins, fats and other nutrients stored in the seed itself.

At the Devonian Botanic Garden, we prefer to sow our seeds in a mix that is 50% coarse sand and 50% peat moss, with a small amount of fertilizer. Like the commercial grade mixes, this sowing medium remains loose and friable and therefore any seedlings are fairly easy to separate when it comes time to pot them individually.

Incorporating an actual soil or loam in with a seedling mix should be avoided as it may result in some compaction, a lack of proper drainage and poor levels of available oxygen to the tender young roots.

Seedling Trays

The size of pot, tray or container should be of sufficient depth to allow good initial root development and it should be wide enough to comfortably accommodate the amount of seeds you intend to sow. As a general rule, most common annual seedlings have fairly shallow root systems and so one can often sow their seeds in a somewhat shallower type tray or pot to start.

Most perennial or tree and shrub seedlings on the other hand, are going to require more depth for the roots. The width of the container used should be relative to the amount of seeds sown and each seed must have its own little individual space around it.

The number one golden rule of sowing seeds is: Thou Shalt Not Over-Sow is an excellent rule to remember.

Over-sowing will produce many weak, spindly seedlings that are all competing against each other instead of fewer but healthier, more robust seedlings. It has been my experience that a maximum sowing rate between 20 to 30 (average-sized) seeds in a standard 4-inch round pot seems to be the correct amount. For really large seeds in the same pot, I would recommend reducing the rate to about 10 or 12 seeds at the most. If you wish to grow a greater number of plants, then just sow in a larger container or use several smaller containers for the same batch of seed.

A good way to prevent over-sowing is to measure out the correct number of seeds into a light coloured or reflective scoop of some kind. An aluminium pie plate cut to fit works really well as a seed scoop. The seeds will be clearly visible on the scoop and once it is empty, you are guaranteed the right amount has been distributed on the surface of the mix. No more, no less.

Now comes the easy method of sowing all those precious seeds you have obtained. Loosely fill your container with the medium to approximately one-quarter of an inch from the top and very gently pat the surface flat with something suitable, like a thin piece of plywood with a conveniently attached handle.

Then, carefully sprinkle the measured amount of seeds evenly across the surface
(a) and press them firmly into the mix, but not so firm as to really compact the medium.

(b) Next, sprinkle the surface with an even covering of an inorganic material such as coarse sand, granite grit or a fine sized gravel and firmly press flat once again.

1. The material must be applied thick enough so that no patches of the sowing mix are visible, but should be not much thicker than about an eighth of an inch. This layer helps to keep the seeds in place when watering so they do not float or slosh around and also keeps the surface of the mix at a more even temperature. Furthermore, it provides the seeds with two very important conditions – darkness and surface moisture retention.
2. A fine, silty type sand is no good for this purpose as it will form a hard solid plug on top.

(c) Labeling each container is a good practice and any label you have written out should include the name of the plant and the sowing date. The date is an important reference point if you intend to sow many different seeds simultaneously. You may want to confirm the germination times later on or check if a particular seed is overdue in coming up.

Now fill a can, sink or tub of cool water at a level about halfway up the side of the pot and let it sit until the entire surface appears nice and damp. This is called ‘bottom soaking’. It allows for a gentle absorption of water from the bottom up, while at the same time the soil settles evenly.

Then remove the pot from the water and drain before placing it somewhere warm and sunny or under grow lights if you have them. In an indoor house type environment without the aid of grow lights in Alberta, a south-facing windowsill at room temperature seems to be the most advantageous spot for a pot of freshly sown seeds.

If you are going to place the pot on top of a saucer or bowl to catch any access water, be sure not to allow the pot to sit in that water for any length of time. Doing so will cause your mix to stay too wet and any seeds will likely rot.

There are some tougher seeds in this world that require some special treatment before sowing. The main event that triggers a seed to germinate is when water penetrates through the outer coating and reaches the embryo inside. Seeds that germinate readily usually have a relatively soft outer shell and so water is absorbed quickly and becomes immediately available to the embryo.

Other kinds of seeds however, possess a much harder and more impermeable coating and may take a very long time to germinate without help. Nature has designed them to remain alive, but asleep for long periods of time in order to survive any unfavourable conditions. In fact, all ripe seeds are more or less in a state of dormancy until the conditions are right for them. We must break that dormancy if we wish to grow them according to our own time frames.

One way to do this is called cold wet stratification and it is simply a way of tricking the seed into believing it has been through a natural winter. There are certainly plant families that contain many different genera which produce seeds that need to be stratified before sowing; The Pine, Maple, Rose, Figwort, Peony, Saxifrage, Carnation, Gentian, Primrose, Lily and Buttercup families are some of the ones under that category.

Do not be fooled into thinking the seeds of every single plant in each of those families should be stratified. One will always find an exception to the rule in life and the plant world is no different.

Stratification

1. Basic stratification can be accomplished by placing a teaspoon or two of sterilized moist sand (or some other inorganic medium) in the bottom of a small plastic zip-loc bag.
2. Pour in your seeds, throw in a few more spoonfuls of the medium on top and carefully seal the bag.
3. Turn the bag over and over with your fingertips while gently squeezing to mix the seeds as evenly as possible with the medium.
4. Attach a dated label to the bag using an elastic rubber band and place in a normal domestic refrigerator.

The optimum temperature is around 4 or 5 °C. The amount of time the bag spends in the refrigerator will really depend on many factors (i.e. type and freshness of the seed) and can be anywhere from a couple of weeks up to 6 months. That is why the stratifying date on the label is quite useful to monitor the ongoing development of the seeds over time.

Cold wet stratifying breaks down the seed coatings resistance and provides it with the three conditions of a natural winter - darkness, cold temperatures and moisture.

The real trick to artificially simulating a winter by stratifying is getting the moisture level of the medium correct. If the medium is too wet, the seeds may rot and die. If it is too dry, nothing will happen to the seeds, and you will fail to break the dormancy. Aim for an adequate moisture level, but not dripping wet.

Once the seeds have been stratified long enough, empty the contents of the bag uniformly over some seeding mix and sow as normal.

If you see things sprouting inside the bag while still in the refrigerator, do not panic. Just gently sow as before and your new seedlings will push up through the top covering soon enough.

Simple Stratification Method

An even simpler method of stratifying is to sow the seeds in a pot, get it good and moist, stick it outside and cover it with ample amounts of snow. Then, bring the pot inside later on to thaw out and water as necessary. The term scarification refers to the practice of physically removing a portion of the seeds outer coating and can be done by rubbing the seeds across a sheet of sandpaper or nicking them with a sharp blade. But whether it is stratification or scarification, the goal of interrupting the hard coating in order to allow water to seep inside is achieved, thus paving the way for germination.

Delicate Seeds

But what about these exceptions to the rules of regular sowing I hear you ask? Well, the seeds of plants like Begonia, Lobelia, Rhododendron and Petunia are extremely small and usually do not take too kindly to being covered with sand or gravel. They prefer to be surface-sowed with little or no covering and then either misted often or bottom soaked as far as watering goes.

Another way of ensuring the tiny seeds do not get washed around and still stay moist is to place a thin sheet of glass over the top of the pot. This still allows for maximum light while keeping the moisture and humidity levels up.

Seeds of Chrysanthemums and Garden Cress (Lepidium) also favour surface sowing at times as they germinate better in total light conditions, instead of under the darkness of a covering.

Cactus is another candidate for surface sowing. The seedlings will sometimes form a mutated ball rather than a proper stem if buried too deep in a medium. The Himalayan Blue Poppy (Meconopsis betonicifolia and M. grandis) and most common Pansies (Viola) on the other hand, can be sown normally in bright light, but will actually germinate much better at cooler temperatures around 10 °C.

Tropical plant seeds as you would expect, like to germinate under really warm humid conditions in full light, which is not the kind of environment one usually finds in a typical Alberta household in the early spring.

Some seeds in the Pea family (Fabaceae) need to be soaked in warm water overnight prior to sowing. There are other exceptions of course, but most commercial seed companies usually include reasonable instructions on the packets if the seed in question requires any special treatment or conditions to germinate.

Growing your own plants from seed is not only cheaper but just think of the pride you can have in showing people all the fabulous specimens in your garden that you personally grew. I know I love doing it, so give it a try if you haven’t already. It is as easy as falling off a log, and much less painful to boot. Well, until next time we meet, happy gardening and I most certainly wish you good luck in conducting your own seedy affair.