Growing and Propagation

MARCH 2020: PROPAGATING FROM SEED

March is a key month for seed sowing with the majority of our summer flowers being started over the next few weeks. Most of the cut flower varieties we grow belong to the group of plants known as Annuals. This includes Snapdragons, Cosmos, Zinnias, Scabious, Statice, Ammi and many, many more. They complete their life cycle within one growing season, flowering through the summer before setting seed to produce the next generation. Because of their short lifespan and the urgency with which they need to produce seed, these are some of the most productive plants in the cutting garden. In general, the more you cut, the more flowers they produce.Most growers have their own tried and tested way of producing plants from seed. There is no one right or wrong way to do this. It’s important that you use a method that suits you and your set up. Here I talk about the process I’ve developed over the years. It works for me and produces strong healthy plants and beautiful flowers but if you have a method that you’re already having good success with, do stick with that. It can be easy to get tripped up by the plethora of advice out there if you try and follow all of it.

We only have limited space for propagating and so growing the right amount of each variety is very important – it saves space and valuable resources such as seed, compost and water. During my planning process, I calculate how many plants I will need of each variety and sow accordingly. Very rarely do I need large quantities of a single variety and so I hardly ever sow into seed trays. Instead I prefer to sow into 9cm pots like this.

I might sow several pots of one variety but this still takes up less space than even a half seed tray. Before I start, I make sure I have all my labels written with the variety and planting location clearly marked.I use Sylvagrow Sustainable Growing Medium, a peat free compost with good reviews. I like it because it’s fine enough to not need sieving before seed sowing. It feels nice to use and doesn’t get waterlogged so damping off (where seedlings rot at the base of the stem) is less of an issue. I do have problems with Fungus Gnats living in the compost and feeding off the seedling roots and this is mostly a problem in my home conservatory where I start the seeds off. The warmer temperatures there seem to suit the gnats but this is easily managed by spraying the compost surface with a mixture of 3% peroxide solution. I use 1 part peroxide to 4 parts water.

I fill the 9cm pots up so that the compost is 1cm below the rim. I use another pot to firm the compost down gently. This is important as is consolidates the medium which helps the roots get a hold and anchor the seedlings.

How I sow the seeds depends on their size. For large seeds, I place these on the compost surface by hand and push them in gently. They’re large enough that I can count how many I’m sowing. For sweet peas, I use a different system, starting them in root trainers instead (see earlier blog post). For smaller seeds, I tend to tap them out of my palm or sprinkle a pinch at a time.

I keep going until the surface is covered evenly.

Then I use another 9cm pot with a small amount of compost in the bottom as a sieve to add a light dusting over the seeds.

Once I’m done, I water with a very fine rose watering can. I particularly like the Hawes 1 pint watering can. The only seeds I don’t water like this are the very fine ones such as snapdragons, foxgloves and poppies. For these, I water the compost first then sow onto the damp surface. None of these seeds need covering with compost either.

I place the pots into a heated propagator along with the cuttings, I’ve been taking, until the seedlings have germinated. I have a couple of all-in-one units like these as well as a separate heat mat. The heat mat takes up less space, can hold more than the electric propagators and has a thermostat I can adjust. I prefer this one for germinating seeds but favour the all-in-one units for rooting cuttings because the lids are deeper so fit better over the taller cuttings.

Once the seedlings have germinated, the pots are removed to a cooler location. It’s important to do so as they only need the 18-20 degree heat of the propagator for germination. If left to grow on in these conditions, they quickly become weak and leggy. Hardy annuals (those that can withstand cold temperatures) are moved to a cold greenhouse. The half-hardy annuals (like these Statice) need frost protection and stay in the conservatory for a few weeks.

Once the seedlings are large enough to handle, they’re ready to be pricked out into a cell tray. Not everyone does pricking out – some prefer to sow into modules that are large enough for the seedlings to grow on in. I’ve never produced as healthy plants with this method. Many seedlings can get quite top heavy if left in their original sowing position. Pricking out allows you to reposition the seedlings in the compost so that more of the stem is supported. This gives me much sturdier plants. For most varieties, I like to prick them out as soon as I’m able to. I find it causes less root disturbance and the seedlings are more likely to establish quickly in their new environment and grow on strongly – this works particularly well for Larkspur which is often recommeded to be sown direct instead. The only exceptions are those tiny seedlings like Snapdragons and Foxglove. If pricked out when they’re too small, they fail to thrive so I leave them a week or two longer than other varieties so they’re larger and stronger by the time I prick them out. I like to prick out into a 9 cell tray. This gives enough space and fresh compost for the seedlings to grow into without risking them being surrounded to by big a volume of cold, wet compost – something that can hinder growth. I fill the cell tray with compost, making sure to press it gently into the corners of each cell. It’s important to do this so there aren’t any large air gaps otherwise, the compost and pricked out seedling will sink as soon as the tray is watered.

I top up the tray with compost and level it off. The surface should be firm but not so compacted that all of the air has been squeezed out – roots need oxygen for healthy growth.

To remove the seedlings from their pot, I like to use a pencil to lever them up gently. I find it’s better to go in at the side of a seedling so that the pencil is lifting it up from underneath. It’s best not to pull the seedlings without doing this as they’re likely to snap off. It’s also important to hold the seedlings by their leaves so that the stem isn’t damaged. Leaves can regrow but a damaged stem is generally fatal.

With the pencil, I dib a hole into the first cell, right down to the bottom.

Then I lower the seedling’s roots into it. It’s important to try and avoid the roots curling up or sticking to the sides. If the root is long like this, it’s actually better to pinch off about 1/4 of it rather than letting it coil up. This ‘pruning’ encourages the roots to branch. The only varieties I wouldn’t do this to are those in the carrot family, like Daucus and Ammi, that produce a long tap root, and those such as Larkspur that dislike root disturbance.

I firm the compost around the seedling and continue until the tray is full. Once I have two completed cell trays, I water them and place them in a seed tray so that they can sit in the seed tray rack in the greenhouse (or on the bench in the conservatory if they need frost protection).

Many varieties are planted straight out into the flower beds once they’ve filled these cells. However, for larger, faster growing varieties such as Cosmos and Ammi, I pot them on into a 9cm pot until it’s time to plant them out.By propagating from seed in this way, we’re able to produce a large number of highly productive plants relatively cheaply giving us a plot full of flowers for cutting all summer long.

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