Sowing in Pots and Punnets

Sowing directly in your food garden means you don't have to transplant seedlings and therefore avoid transplant stress.  So why then can it be better in some cases to sow in pots or punnets inside or in a hothouse?  This blog post looks at why it can be a good idea to sow in punnets and how this is best done.



But first .......... Why sow your own when you can buy ready-to-plant seedlings?

• Sowing your own seeds is fun, if you stick to a few rules that are explained below!  Seeing a plant grow up from seed to crop is a great experience.
• Yes, there are millions of seedlings for sale, but chances are that the variety that you like to grow, is only available as seed.  Sowing-your-own gives you access to a much broader range of varieties of whatever vegetable you like to grow.
• Sowing your own is lots cheaper.   It will cost nothing if you use seed your harvested from your own garden the previous season.

Some vegetables (for instance radishes, peas and beans, carrots and parsnips) will have a stressful life and therefore not become a real success, if sown in punnets, and then later planted out.  They should be 'sown in situ'.  Sowing in situ (straight into your garden soil) is covered in an earlier post on this blog.  Many vegetables can be sown in situ, or they can be sown in punnets and then planted out.

Why sow in punnets rather than straight into the garden?

• If you have your punnets inside or in a hothouse they will be in a much warmer environment.  You will gain valuable weeks or months by sowing inside and then transplanting when the weather is warm enough for plants to be okay outside.
• Because it may give you more control over the germination process and therefore more successful, viable seedlings.

In Tasmania, a good example is tomatoes.  You could sow tomatoes directly in your garden, but very few people in Tasmania do, because before November soils will be too cool and you would have very little success.  Instead, tomatoes are often sown inside in early September, and planted outside in November.

What are ideal circumstances for seeds in punnets?

• Well draining soil. The best soil for germinating seeds remains moist when watered, but excess water drains away.
• The soil does not contain large particles, so there are no air pockets. Seeds are surrounded by soil.
• Moisture levels are kept constant.
• Temperatures are even and for most seeds ideally between 18 and 22 degrees.  Seeds like pumpkin do not germinate unless the temperature of the soil is around 21 degrees.
• Food is available either via the seed-raising mix or via water added to it.

Now compare the two photos below:

Tomato seedlings sown a fortnight ago

The same tomato seedlings a month after sowing

I often sow two tomato seeds per pot and then remove the least successful of the two seedling. The four tomato seeds sown in the pots in the photo above are all from the same batch of seeds. The two punnets were together the whole time.  I treated them the same way all the time.

The only thing that was different was that the pot on the left was filled with not-so-cheap seed-raising mix of 'a good brand' bought at a hardware store, whereas the punnet on the right contains a not very sophisticated seed-raising mix I made myself.

The first photo was taken just 14 days after the seeds were sown, but the difference is already clear. The second photo was taken a fortnight later.

Whatever ready-to-use seed-raising mix you buy from a nursery or hardware store, it is best to assume that this mix will be very much lacking in nutrients, even if, as was this case here, the brand claims that the mix 'contains controlled-release plant food'.  On the bag it also says 'for better results, after the first two leaves have emerged, begin fertilising with a liquid plant food'.  I did that for both pots.  This experiment tells me that in this case the commercially produced seed-raising mix is really a recipe for failure.

Should I do anything when seedlings emerge?

Irrespective of what seed-raising mix I use, I fertilise once a week with liquid plant food once seedlings have emerged.  I use PowerFeed (contains fish emulsion and seaweed extract) by the makers of Seasol, but other brands of organic liquid fertiliser might be equally effective.  I add the fertiliser (in the recommended ratio) to water that I put on the soil.  I avoid watering on leaves as I have my punnets permanently on a sunny window sill and leaves could get burned if they become wet.  Both punnets in the photo above received this treatment once a week. 

How do I make my own seed-raising mix?

Most gardeners who makes their own seed-raising mix will make variations of the following:

• 3 parts sifted compost  - to provide nutrients
• 1 part (after dissolving it in water) coir fibre - to retain moisture and provide aeration
• 1 part vermiculite or course sand or perlite - to improve drainage and aeration
• 1/2 part pulverised cow or sheep manure or worm castings - helps with drainage and nutrients

Mix these ingredients well together and, hey presto, you have a terrific seed-raising mix.

Get yourself a sieve, or make one, that is just fine enough, but not too fine, so you can make yourself a fine compost mix from the rough mix that comes off your compost heap.

Most homemade composts are superior to any compost you buy. However, they might contain seeds that might compete with the seeds you are trying to grow.  Homemade compost isn’t sterile. It can contain bacteria and fungi that can in theory kill your seedlings. My non-sterilised home-made compost has never created problems for me, but you can sterilise compost by cooking it in a slow cooker until it smells cooked. Then let it sit for a couple of days before using it.

Coir fibre made from coconut or peat (I prefer coconut because peat may be dug up, possibly destroying the environment) is very interesting stuff. Put a block in half a bucket of water and the stuff swells to an enormous size: coir has an excellent water-holding capacity.

Coarse sand is cheaper than vermiculite and perlite, although none of these are particularly expensive. Vermiculite is a natural volcanic material. Unlike sand, it is light and retains moisture and minerals. Perlite is volcanic rock that has been heat-treated at a high temperature to produce an extremely light material that is ideal for propagation, hydroponics and seed-raising. 

Vermiculite (left) and Perlite (right).  The pea tells you what size the particles are.

Seeds do not need food to sprout. However, once your seedlings start growing, they do need food. Avoid the need to replant in another mix, and therefore risk transplant stress, by adding a small amount of pulverised cow or sheep manure to your mix.

Mizuna seedlings in home-made seed-raising mix with Perlite

How do I go about the actual sowing?

1. Choose your pot(s) or punnet(s) well.  If you expect that it will take 4 or 6 or even more weeks before you plant out your seedlings (for tomatoes for instance 6 weeks or more would be normal) don't choose empty toilet rolls or something of similar small size, because small tubes of soil easily dry out and roots may have nowhere to go.
2. Completely fill punnet or pot with seed-raising mix, level it off with your hand, but don't compress.
3. Bump the punnet a few times against a table surface: that compresses the soil enough.
4. Small seeds: lightly sprinkle them over the surface.  With bigger seeds go slightly deeper.
5. Now put a thin layer of vermiculite over the soil.  This holds the moisture really well.
6. Put the punnet in a warm sunny position and water with (ideally luke-warm) water once a day.
7. Until seedlings emerge above ground, warmth and moisture are their most important requirement.  Ideally keep temperatures even and around 18 - 22 degrees.  In winter you can achieve this by putting the punnets on a heat mat under a plastic enclosure (see photo below).
8. When seedlings appear above ground the most important requirements become light and food.  If you have the punnet on top of a heating mat, move it out of that situation to a nice light, not too cold spot (a north-facing window sill is great).  Keep the seed-raising mix moist but not wet, and water once a week with liquid plant food mix.
9. When most seedlings have their first set of adult leaves, remove weaker seedlings, thereby creating more room for the remaining seedlings to grow.
10. A good time to transplant the seedlings is when roots begin to show through the bottom of the pot or punnet.  At that time you could harden off the seedlings, which means allowing them to get used to the outside environment before planting.  For a few days put the punnet outside for a few hours during the day in the spot where you soon are going to plant the seedlings.
11. Plant the seedlings at the same depth as they were in the punnet, except if they have become a bit leggy.  In that case plant them a bit deeper.

A heating tray with inbuilt heating element

Happy sowing!