How to Sow Seed Indoors
You will need
Most containers will work, as long as it has holes or cracks in the bottom for drainage, although it is best to avoid wood since it can harbour diseases. Seed trays are particularly good, and can be bought cheaply from your local garden centre.
You can reuse containers every year, but it is best to sterilise them with jeyes fluid. If the hole in the bottom of the container is large, I think it can help to put broken pieces of crockery or large stones in the bottom.
The other consideration as far as containers are concerned is that earth based containers are much better at holding heat, and breath better, than plastic. Terracotta containers are better for young plants than plastic containers. But they are harder to clean, and so professional nurseries have moved over to plastic.
A seed compost is generally best, although I have used multi-purpose compost without any problems for years.
There are a number of theories with seed compost. One of these is that adding a few handfuls of garden soil into each bag will allow plants to get used to the microbes in your garden soil. I’ve never seen any real effect from this.
In reality, peat based compost is the best, in that it is consistent and holds water well. However, peat is a non-renewable resource, and when you pot the seedlings on a mixture of non-peat compost and garden soil will work very well.
You can collect your own seed from many plants, or buy from a supermarket or garden centre. Most seed should be freshly bought every year for best results.
Seed is best stored cold and dry. Perfectly dried seed will store in a freezer for year, but if you buy shop bought seed I suggest the best place to store it is in your fridge, in a cardboard box rather than metal.
Native seed is great, because you can generally collect it.
Hybrid seeds often have a lot of vigour and produce great results, but the cost is that they do not come true, so you will need to buy seed every year.
4. Watering tray
With most seedlings, it is better to water from below, because drops can damage the seed.
A watering tray is therefore really important. I think plastic trays are best. My view is that at the early stages of plant germination the tray should be shallow, and as plants get bigger, you start needing deeper trays.
How to sow
5. Put soil into container
It is best to make the surface of the soil roughly even, and a little bit below the top of the surface. Most seed works best with at least an inch of soil.
The exact details of depth vary depending on what it says on the seed packet. Before you sow the seed, I recommend pressing the soil down quite firmly so that there are no air pockets in the seed bed.
6. Dampen the soil
It is important not to let the soil get more than damp to the touch, because if the soil is too wet the plant will rot. Regular but small watering is important during the early stages of germination. As the plant gets bigger it is generally more robust and will cope better with variations of water feeding.
The water should be warm to the touch... if you would give it to a baby, it will work really well when you feed it to your seed. After all, seed is a baby plant!
7. Soak some seeds
If the seed is a big seed like beans or peas, it can be good to give it short soak, to start the germination process. Small seeds are better sown without soaking.
Make sure you read the packet, because different seeds sometimes need weird or unusual conditions to germinate. You need to nick sweatpeas or they will never germinate, and there are some weird and wonderful seeds that need certain heat conditions to germinate... for example, they need to be frozen and then warmed up for a certain number of days.
As a rule, most vegetable seeds are quite forgiving. However, when you soak the seed it is very important to use lukewarm to warm water, since if the water is too cold it will retard germination, and if it is too warm it may even kill the plant!
If it feels like warm spring rain, it is perfect!
8. Depth of planting
Check the packet for details, but the rule of thumb is that if seed is small, you shouldn’t cover it. Other seeds should be covered with one or two times their width in compost.
This is really important, and it is worth checking out the depth of planting on the back of the seed packet! However, a rule of thumb is to sprinkle very fine seed on the surface, larger seeds twice as deep as they are tall, and sprinkle the top with a layer around 1/4 of a centimetre deep with fine gravel bought from a garden centre specifically sold for seeds.
Once sown your plants will need
Most seeds will germinate on a windowsill, basically the same heat as a nice spring day. Put them into a windowsill or a heated propagator. If you keep them on a windowsill, chose a north facing one, and ‘turn’ them every regularly every day once they have germinated. This stops them growing towards the sun.
Once they have germinated, give them water regularly. The soil shouldn’t be soaking wet, but allow it to be damp. The water should be lukewarm to the touch.
11. Pricking out.
When a seedling germinates, it starts off with a couple of seedling leafs. Once a ‘true leaf’ develops – that is, leafs that look more like the actual mature plant – it is time to prick the seeds out and plant them in a seed tray.
This involves using a small plant label or dibber to gently lift the soil around the plant. Be careful not to break the plants roots or leaves.
As the plant grows, it may get too large for the container, in which case you will have to pot it onto a larger container.
13. Hardening off
After the plants start to get big and bushy, and you are no longer experiencing frosts, it is best to put it outside during the days for a week, and then leave them in a cold frame until you are ready to plant them out.
You can read more of my thoughts on hardy perennial