Potting on: sunflower seedlings

Notice the significant difference in height in the seedlings after only 16 days. Each seedling is competing for light & space above ground and water & nutrients below ground

Sixteen days ago, my glamorous assistant (my wee mum!) and I sowed 50 ancient old sunflower seeds in  close drills in a single seed tray, assuming a really low germination rate due to the age of the seeds. Lo and behold, 41 out of 50 popped their little heads up! Despite the dizziness of my vertigo this last few days, I’ve been aware that the rapidly growing seedlings really needed potting on ASAP; else they will become irrevocably stunted before reaching the next stage of their development. The equivalent of falling at the first hurdle!

cotyledon leaves and first true leaves in a sunflower seedling

As seen from the above photograph, each seedling has put on a fairly huge whack of roots under the surface, alongside a good 10-15cm upwards growth. Most seedlings* we would expect to see growing in a garden or allotment  start the same way; they push up two ‘baby leaves’, called cotyledon (Latin trans: ‘seed leaves’). These leaves tend to have a large surface area comparative to the size of the seedling,  mopping up as much sunlight, and therefore energy, for the seedling as possible vis-à-vis photosynthesis. They function like solar panels. When I started out, I remembered cotyledon as ‘cot lead on’ and thought of the baby leaves as like a baby’s first teeth. It helped me both remember the term and relate to it! Next come the ‘true leaves’, which are the first leaves of the new plant. Please note the significant distinction in the colour, size, texture and shape between the cotyledon and true leaves of the sunflower seedling in the above photograph.

Potting on is a really important step for tender and fast growing seedlings. In general I prefer to wait until at least 2 sets of true leaves have appeared on the new seedling before potting on. Usually I would plant sunflower seeds in their own pot, rather than a shallower seed tray, which would delay potting  on into May. But needs must, and so yesterday we potted on all 41 of the sun-worshippers.

Potting on is one of two vulnerable steps in our seed’s journey from it’s germination in a relatively controlled growing environment (in this case, in seed trays in a south-facing window); to the big, wide outdoor world full of predators (slugs, snails, hungry insects, birds & mice!) and climate (wind, rain, fluctuating temperatures!). It is a vulnerable step as it’s when our seedling’s stems are at their most tender, and thus most susceptible to damage. Given that the stem is essentially an extremely efficient and sophisticated transport system moving water and nutrients up and down the plant,  if it’s damaged at this stage, the odds are that sadly the seedling will not survive, or will be stunted. The second most vulnerable step on the journey is ‘hardening off‘, which I’ll explain in a subsequent blog post when we get to that stage.

Therefore, the number one rule for potting on is to never, ever,  hold seedlings by their stem. Best is to hold them by the scoop of compost at their roots, or, at a push, by their leaves. During this coronavirus lockdown we are limited in the garden resources we can find and this extends to compost. For potting on I would always prefer a decent non-peat based specialist compost for seedlings. This ensures that the growing medium is neither too rich or too poor, has a texture open enough for seedlings to continue to strengthen their roots, whilst also firm enough to hold them in place. Think Goldilocks. It is about providing the optimal growing conditions at the right time for the plant’s development. But, again, needs must and, this year  my potting on compost is  a general multipurpose compost mixed in with some of my fantastic kitchen compost. My neighbour kindly gave me nine 50 litre bags of compost the other day.

This green bin with a lid stores around 100 litres of compost. I find it easier to fill from a bin than from a bag. Less mess.

In the photograph above, my glam assistant is half filling the pots with the compost mix. I  then popped a seedling in each half filled pot (e.g bottom left)  and then filled up the pot, taking care not to damage the stem (e.g bottom right). I decided to partially bury the stems  at this stage to give them a bit more stability in the pot. In a week or so I will add little support sticks for them too. With particularly leggy seedlings you can pot them on submerged up to the cotyledon leaves. I generally do this with seedlings such as brussel sprouts, purple sprouting broccoli, cabbages, pumpkins, cucumbers; and any other large plants which will bear a lot of weight come maturity. It helps them develop a strong base from the get go.


using a little potting station keeps mess at bay. A basin or cardboard box would do too.

We potted them up in a mixture of sized pots, some 7cm wide and deep, others 12 cm wide and deep (what was available). After giving them a really good soaking, with the rose on the watering can turned up (which gives a gentler water pressure than with the rose turned down), I placed them inside temporary cold-frames (mini-greenhouses). Normally storage boxes, these boxes are brilliant for temporary cold-frames as they are stackable and moveable. I first saw them used on canal boats and thought them a fantastic idea.

IMG_20200422_165237395_HDR (2)
sunflower seedlings in the boxes and tray with 6 pots. In the foreground are potted on courgettes and to the left are potted on pak choi I’m growing for seed.

For the next few weeks,  I’ll carry them outside in the morning, to let them soak up the sun, and in the evening I’ll bring them inside, stacking them next to the window. You can either put the pots on the lid on the floor, and so put the box on and off like a cloche, or just put them inside the box. I’ve opted for the latter whils’t I’m moving them in and out the house. Once I move to the second vulnerable stage, of ‘hardening off’ the seedlings, I will probably move the seedlings to sit on the lid and use the box as a cloche….but that is for a few weeks hence!

Below is a picture of the boxes stacked at the window taken from the outside looking in.

IMG_20200422_175005570_HDR (1)

* The classification of plants which give rise to two cotyledon leaves are called eudicots (eudicotyledonous), or dicots (dicotyledonous). Seedlings with singular seed leaves are called monocots (monocotyledonous). Dicots cover most, but not all of the seedlings typically seem in garden grown fruit & veg. I’ll explore botanical classifications in more detail in another blogpost.

Published by theplothickens

passionate about permaculture, interested in sharing my experiences of designing my allotment using permaculture principles

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