It will sound like I’ve gone a bit over the top when I tell you I was excited to find a packet of a hydrous phyllosilicate mineral when I went to the garden shop looking for some pumice sand. So let me explain.
I was recently given a phial of seed from an interesting native plant found only in a reserve on the flanks of Taranaki, and I wanted to make sure I could sow it properly. The seed was from a variety of rata, and like most of its fellow rata, and its cousin the pohutukawa, the seed is very fine.
As a general rule, seed should be covered at the same depth as its size. Very large seeds must be sown deeply, while others need very fine, or even no cover. As the seed is very fine, and the sowing mix was quite rough, I thought I’d pop along and grab a bag of fine pumice sand. I usually have some around, as I use it to make up mixes for cuttings, but I must have used up the last of the bag.
When I went looking for the pumice, I could not see any, but to my joy I found a bag of vermiculite, the hydrous phyllosilicate mineral mentioned above. It is a mineral that expands and exfoliates when heated and has the ability to retain a lot of water when moistened. It is sometimes used in soilless plant mixtures, and as a hydroponic medium, but the primary horticultural use I have come across is as an aid to germination.
The finer grades of vermiculite are perfect for covering fine seeds – when we owned a bedding plant nursery, almost all the seeds were germinated using a standard potting mix in trays [or flats as nursery workers call them] then covered with a layer of vermiculite.
For those we are raising seed inside, it is generally just a matter of making out a good seed bed in a tray or a pot, then sowing the seed and lightly covering it.
It is a good trick to water the mix well once it is in the tray before sowing, then just giving the newly sown seed a light watering. After that, just keep a light watering regime up until you see the new leaves popping through the surface.
Let the plants grow on until they have their first true leaves, before pricking them out into containers for growing on. Obviously, in the case of the rata plants which will be small when they germinate, I will let them grow to about 2cm high before pricking them out.
I have never grown rata from seed, but some years ago grew some pohutukawa for a friend, and they germinated like hairs on a dog’s back. They needed to be carefully extricated from each other, but once they were potted
on, they grew very easily.
Most of us will only be growing vegetable or bedding plants from seed, and fortunately for us, they have been selected over multiple generations for ease of germination. However, although many plants are easy to grow from seed, there are some that have quite definite tricks to ensure germination.
Some need stratification – they are native to areas that have defined cool periods over winter, and it would be dangerous for the seeds to germinate before spring, so they wait until they have been cooled for a period. You can trick seeds into thinking that had happened by placing them in a refrigerator for a week or two. Those who live in very cold climates can always just pop the containers outside of course.
Oddly, even some Australian plants seem to need stratification, including some Eucalypts – the Head Gardener was not happy about having tubs of those seeds in the refrigerator!
At least one peony species takes this process to the extreme – it needs double stratification. After one cool spell, the roots will develop, but the seed then requires another before the shoots begin to grow.
Other plants need a lot of water to flush out germination-inhibiting chemicals. Plants that grow in areas that have sustained droughts have evolved to only germinate after rain. To trick them we need to have a stream of water passing through the seed for a week or two before they will germinate.
One of the irises I grow is a bit like that so I carefully put the seeds in mesh bags and place them in the cistern of the toilet. A few weeks of intermittent flushing is enough to trick them into thinking it is time to sprout!
Other seed will only germinate within a narrow temperature range – we used to germinate some of the most expensive telegraph-style cucumber seeds by placing them in a damp sack and sitting the sack on a heated bed. We needed to keep a close eye on them to make sure we potted them up as soon as they germinated, as they got tangled in the hessian!