The joys of raising plants from seed

As a garden writer, I’m often asked for advice about raising plants from seed – and I get it. For new gardeners especially, it can feel like a perplexing, frustrating and mysterious process akin to being a codebreaker where horticultural terms such as “hardening off” and “cold stratification” might as well be written in hieroglyphics or ancient Greek.

Crack it, however, and you gain entry to a vast world of possibilities where the range and variety of plants that you can propagate is many, many multiples of what’s available in any typical garden centre or nursery. Raising your own plants from seed is also much more cost effective, especially if you have a new or large garden to stock. It is also joyful in a way that never grows stale.

One of the first things to make me look very differently at the process was the discovery that seeds are alive. It seems obvious now. Of course they are, how could they not be? It was a revelation, however, to find out that, much like the princess in Sleeping Beauty, they exist in a state of almost suspended animation, dormant, respiring at a rate just barely enough to sustain them while waiting for the kiss of life.

For most, that magical “kiss” is a combination of three things – heat, water and time. The fourth vital ingredient is a degree of darkness or light. Exactly how much of each is required to make a seed germinate is a sort of finely-tuned horticultural recipe that varies according to the particular species.

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Tomato seeds, for example, need darkness, humidity, five to seven days and lots of heat (ideally 23-29 degrees) for a fast, high germination rate. This is not surprising when you consider that the species hails from tropical South America. While it also needs darkness and moisture, sweet pea seed needs approximately 14-21 days and cooler conditions (13-18 degrees) to germinate. Foxglove seed, however, needs light and moisture for approximately 14-21 days at a temperature of 18-21 degrees.

Some species also need the trigger of a period of cold temperatures to jolt them into germination. In nature this happens over winter, sometimes beneath a layer of snow, which is a process known as cold stratification. As gardeners, though, we can artificially reproduce that process by placing the seed in a plastic bag of damp compost or vermiculite (see below) and putting this in the fridge or freezer for several weeks.

This method works brilliantly for certain tricky-to-germinate species including delphinium (one week in the fridge at 4 degrees, then 10-20 days at 18-21 degrees) and Bupleurum longifolium (two to four weeks at 18-22 degrees, then four to six weeks at minus 4 to plus 4 degrees, then back to gentle heat of 5-12 degrees). It’s a bit of a faff but magical when you get it right.

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Both the depth and density that seeds are sown at will also affect successful germination. Sow too thickly, for example, and you dramatically raise the chances of “damping-off” disease striking down emerging seedlings as well as forcing them to compete for light, moisture and nutrients. Sow too deeply and it’s a struggle for the seedlings to emerge but sow too shallowly and they’ll struggle to anchor their baby root systems.

As a guide, the smaller the seed, the shallower you should sow. Really fine seed – for example, the tiny seeds of poppies or snapdragons – needs to be surface-sown and barely covered with a fine layer of vermiculite or compost.

For all the reasons outlined above – as well as to protect tender and half-hardy species from cold temperatures, to give them and hardier species a substantial head-start and to protect baby seedlings from slugs and snails – I prefer, where possible, to sow seed indoors or under cover of a glasshouse or polytunnel at this time of year. The only exceptions are species that are best direct-sown in their final positions outdoors in the garden.

Rather than seed trays, I use three-litre pots. Almost every gardener will have a stash of these pots leftover from plants purchased in a garden centre. These are easier to keep watered, better for root development (they’re great for seedlings with larger, probing root systems) and very space efficient, especially if I need to give them some sort of bottom heat in the shape of an electric propagator, soil warming cable or grow mat (suppliers for these include mrmiddleton.com, quickcrop.ie and fruithillfarm.com).

Of course, seeds sown this way need a really good quality growing medium to sustain and anchor them – one that’s free-draining and friable. My go-to is Klasmann’s peat-free organic compost. It is a joy to use with a very fine particle size and just the right balance of nutrients for baby seedlings (available from fruithillfarm.com). To help preserve air and moisture in the top layer of compost throughout the germination process and prevent “damping off” disease, I also work some fine vermiculite into the top 5cm before sowing. Available from most good garden centres, this lightweight, sterile and naturally-occurring mineral helps with root development, drainage and temperature regulation (as no baby seedling likes to sit in cold wet compost).

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To boost humidity levels, I cover freshly-sown pots with an upturned five-litre clear plastic freezer bag to make a sort of mini-polytunnel or biodome, secured in place with an elastic band. Rather than watering from above, which runs the high risk of washing tiny seeds off the surface of the compost, each pot is placed up to its shoulders in a deep tray of tepid water, which is called bottom watering, and left to soak for a few hours. Each pot also gets a label marked with a fine waterproof pen listing the name, date sown and seed supplier.

Whatever the species, once that magical process of germination occurs, the conditions required by the seedlings for optimum growth rapidly changes once again, like a kaleidoscope that’s been shaken. As a rule, they now immediately need less heat, much lower humidity but lots of natural light.

At this point it’s vital to immediately start ventilating the pots by gradually easing off the plastic bags and removing them from any source of bottom heat. If you don’t have a sheltered, frost-free spot in a glasshouse or polytunnel then a bright porch, conservatory or windowsill is fine.

Keep them lightly watered and then in a few weeks, when their true leaves emerge, prick them out. Do this by using a pencil or stick to gently lever each individual seedling out of the damp compost, always holding it by its leaves rather than its vulnerable stem. Then, gently lower its root system into either a small pot or a cell in a cell tray filled with a good quality potting compost. Gently firm the compost around it, water lightly and regularly but only ever enough to keep the compost very slightly damp and never letting it dry out. A very diluted liquid seaweed feed given every week will help to keep your baby plants hale and hearty until the time comes to plant them outdoors.

This week in the garden

The fierce heat and droughts of previous summers may seem like a different universe but now is a great time to generously mulch established flower and shrub borders to lock in moisture and protect their root systems from heat stress. You can use well-rotted farmyard manure, homemade garden compost or a commercial product (see mulch.ie, envirogardenandhome.com and geeup.ie) to do this. Just make sure to keep the mulch out of direct contact with the woody stems of trees and shrubs.

Tomato seedlings raised from seed sown last month need to be potted on to prevent checks to their growth and produce strong and healthy plants. If yours look spindly with weak, floppy growth then, along with needing to be potted on, they’re almost certainly getting too much heat and not enough light. Very spindly plants can be re-potted with their root ball buried lower in the compost along with a section of the base of the stem as a way to correct that weak, etiolated growth and produce a strong plant.

Dates for your Diary

Tuesday, March 29th (8pm), Parade Tower, Kilkenny Castle, ‘Wildlife Lessons from the Garden’ , a talk by the ecologist and plantsperson Roger Goodwillie as part of this year’s OPW ‘Garden Talks at the Parade Tower’ spring programme, admission free to all, see kilkennycastle.ie

Saturday, April 1st (1.30-4pm), Cabinteely Community School, Johnstown Road, Cabinteely, Dublin 18, D18VH73: Alpine Garden Society Annual Plant Sale with a wide selection of alpine, rock garden and woodland plants, bulbs, ferns and small shrubs for sale as well as stalls by specialist nurseries (Admission €3)

Sunday, April 16th (10am-4pm), Leitrim Flowers, Anamadu Fields, Kilnagross, Co. Leitrim: Introduction to Designing a Cottage Garden, a one-day workshop costing €95 including lunch and refreshments. Pre-booking is essential, see leitrimflowers.ie.