You’d never guess by looking at a seed that it’s alive; that suspended in a state of dormancy, it waits for a certain set of environmental conditions to spur it into growth. Those triggers typically include a mixture of light, humidity and gentle heat, hence the magic of electric propagators and covered growing spaces such as glasshouses, polytunnels, cold frames or simply a bright windowsill.
But for others, it’s more complicated. In the case of primulas, for example, temperatures over 18 degrees prevent germination. Other species need exposure to a period of cold or darkness, or several cycles of cold/heat. For a few that are native to parts of the world where forest fires are a part of the natural ecosystem, it can be stranger again. Germination of acacia seed, for example, is triggered by a mixture of extreme heat and smoke.
For Ireland’s native foxglove, whose seeds can be buried in the ground for years like minuscule time capsules, direct exposure to sunlight – that all-important kiss of life – is what’s needed to jolt it into germination. But in the case of Ireland’s wild orchids, the presence of certain fungal mycorrhizae in the ground is required, a symbiotic relationship where the orchids then return the favour by supplying them with vital carbohydrates.
Every square metre of soil also has its distinct seed bank, botanical fingerprints of seasons past and the plants that once grew there, or whose seed was carried by animals, wind, or by humans. We can unknowingly carry them on the soles of our shoes, for example, or on clothing, or hidden in the compost of a container-grown plant or one given by a gardening friend.
Even the tiniest represents a universe of possibilities, an invitation from nature to collaborate. In this way, seeds are love letters to the future, a promise that life endures. If you’ve ever saved some from a much-loved or coveted plant, then you’ll know the thrill of coaxing them into growth and why it feels almost like a conversation of sorts. If you’re a dedicated seed-saver, then you’ll also know the joy of sharing them with fellow gardeners, an act of ever-spiralling possibilities that allows great garden plants to travel the world. As someone who loves the process, my home is filled with hastily labelled envelopes of seed harvested from the garden or given by generous friends.
That said, at this time of year – the beginning of the busy spring seed-sowing season – I start to reluctantly cull them in the knowledge that not all species are equal when it comes to the longevity of their seed. Tomato seed, for example, remains viable for many years as does that of many kinds of peas and beans, especially when properly stored in a cool, dark, dry space. But not others.
Among the most short-lived are umbellifers (Apiaceae), a large group of plants with umbrella-shaped flowers that includes carrot, parsley, coriander, parsnip, celery, fennel, orlaya, cenolophium, bupleurum, and pimpinella. The viability of their seed naturally quickly decreases over time so needs to be sown as fresh as possible.
Some umbellifers also particularly benefit from exposure to a period of cold to spur them into germination. If you’ve ever, for example, struggled to get Orlaya grandiflora to germinate, then try popping the freshly sown seed of this hardy flowering annual in the fridge for a week or two. Just make sure to lock in moisture first by covering it with a clear lid or bag.
The same requirement for a period of exposure to cold also goes for the seed of many other kinds of popular flowering plants including: larkspur (Delphinium consolida); perennial delphinium; campanula; bells of Ireland (Molucella laevis); astrantia; and rudbeckia. Where space is tight, pop the seed into a clear, watertight freezer bag alongside a small amount of damp, good-quality seed compost before sealing it and putting it in the fridge.
Whichever method you plump for, make sure to remove the bag/tray as soon as you spot signs of germination. If you’re using the bag method, then very gently empty the compost on to a pot or tray already filled with compost and lightly water it to encourage the freshly germinated seeds to sink their baby roots downward.
The Ziploc method is also a space-saving way to test the viability of seed that you suspect may be past its best but are loath to dump. Just leave out the fridge part (unless it’s a species that requires it) and place the bag in a bright, warmish space under cover, checking regularly for signs of germination.
In the case of popular umbelliferous species that are typically direct-sown outdoors such as carrots or parsnips, it’s also crucial that the soil is prevented from drying out or capping while the slow process of germination takes place (this can be up to four weeks). One useful tip is to place a plank of wood over the freshly sown seed, inspecting the ground beneath it regularly for signs of germination and then removing it immediately once you do.
Another excellent way to help the seed of tricky species to germinate is by pre-soaking them overnight in a very diluted solution of liquid seaweed feed. The latter is rich in natural plant growth hormones including auxins, cytokinins, and gibberellins that both speed up germination and increase the percentage of seeds that successfully germinate.
Finally, bear in mind that many (but not all) reputable seed suppliers give a clear indication on the seed packet of the expected viability of the seed contained. While there’s still a good possibility of germination after the cut-off date indicated, the rate of germination (the percentage of seeds that germinate) quickly reduces from then on.
This Week
Check dahlia tubers overwintered under cover and gently remove any rotten segments. While it’s too early in the growing year to plant the tubers outdoors, they can be potted on into large pots/trays and grown under cover in a frost-free glasshouse, polytunnel, conservatory or sunny porch to produce plants that will be many weeks earlier into flower. Recommended Irish suppliers include all good garden centres and Mayo-based gardenblooms4u.ie.
Finish pruning late summer and autumn-flowering (Group 3) clematis, using sharp, clean secateurs to cut the slender stems back to about 15cm above ground level and cutting to just above the nearest set of strong buds. Then finish with a few generous sprinkles of a slow-release organic pelleted fertiliser around the base of the plant followed by a generous organic mulch of well-rotted manure or home-made compost.
Dates For Your Diary: Sunday, March 23rd (11am-4pm), RHSI Bellefield Gardens, Shinrone, Co Offaly, RHSI Bellefield Plant Fair; A Boutique Gathering for Plant Enthusiasts! with a range of specialist nurseries taking part, see rhsi.ie; Tuesday, March 25th (8pm), Foxrock Pastoral Centre, Church Of Our Lady of Perpetual Succour, Foxrock, Dublin 18, Leaf Lust, a talk by garden designer and broadcaster Conrad McCormick on behalf of Foxrock & District Gardening Club (foxrockgardenclub.com).
