We are now moving into the time of year when we start to sow seeds (for those of you not reading this article ‘hot from the oven’ it is the beginning of February) so I figured it may be helpful to write an article on seed germination. We sow hundreds of thousands of seeds a year to supply our vegetable seedling range so get used to the likes and dislikes of a broad range of vegetables. The good news is it is all pretty easy really, after all the seeds actually want to grow, we just have to help them along. I have arranged the following ground breaking article in a Q&A format even though I’m asking and answering the questions myself, no doubt you will find it fascinating…
Can I use the seeds left over from last year?
A good question, I am glad you asked that. Fresh seed will loose its ability to germinate (viability) over time. How long your seeds last largely depends on how they are stored but some seeds will naturally have a longer shelf life than others.
Seeds loose their viability more quickly under damp or hot conditions so it will come as no surprise to learn that they should be stored in a cool and dry place. Seeds will last longest if kept below freezing as for every rise of 5˚C over zero storage life is halved. You don’t have to go to the extremes of the Svalbard Global Seed Vault in Norway above (which has been bored into an arctic mountain) but if you have room in the fridge you can keep them in an airtight container alongside the remains of the shepherd’s pie.
Moisture makes a big difference in the life of a seed so airtight containers are important if you want them to keep for long periods. Using silica gel sachets to absorb any moisture in the container is best way to ensure they stay perfectly dry; the gel can be dried periodically in a low oven and used again. You often get silica gel sachets in product boxes, keep them and ask your friends to keep them, they are very handy!
Some seeds will also keep longer than others no matter how you store them; parsnips for example need to be used the year you buy them while a properly stored pea seed will keep for up to 10 years. As a rule of thumb tomatoes and peas will last 10 years, cabbage family (brassicas) and lettuce will last 5 while root vegetables are usually the most short lived at 2 years.
How do I know if seeds are viable before sowing?
Unfortunately the only way of knowing if a seed is viable is to see if it germinates. You don’t need to go to too much trouble and can do a test germination of a sample of seeds on a wad of moist kitchen paper. Place a few layers of absorbable paper in a shallow dish and sow a few seeds on the surface, cover with another layer of paper and moisten the wad with water. Cover with a plastic bag (to retain moisture) and leave somewhere warm at around 20˚C.
Most seeds need only heat and moisture to germinate so you you can use an airing cupboard, usually we would be worried about emerging shoots getting leggy but as this is only a germination test it doesn’t matter. The most common exceptions here are lettuce and celery which do need light, in this case a warm windowsill is fine.
Once the kitchen paper is kept moist the seeds should germinate in a week or two, if not you will need to purchase new seeds. Remember that seeds will have different germination times so it is worth checking before you throw a tantrum and fling your sodden kitchen paper across the room.
Most vegetables will germinate in 7-10 days but you can double that for carrots and treble it for celery. Here’s a link to a handy germination table giving approximate times for most crops you are likely to grow.
What temperature do I need for germination?
Seeds have an optimum germination temperature which will give you the highest possible germination rate, they tend to be a lot higher than you might think (cabbage 30˚C, carrot 25˚C) but in most cases we start seeds lower down the scale. I tend to take more notice of the minimum temperature as having propagators set at the optimum for the wide range of crops we grow uses a lot of energy and just isn’t practical.
If you are sowing smaller amounts in a heated propagator you can get closer to the optimum temperature but obviously this can be tricky if you have a number of different types of seed on the go. In general, a higher temperature will mean faster germination as seeds need to absorb 40-50% moisture before they germinate and the warmer they are the more they absorb.
We tend to take a happy medium which most professional growers use. Dermot Carey told me he uses just two settings of 18˚C and 22˚C for everything and this is what I stick to and find works very well. We use 18˚for all the crops you would normally grow outside and 22˚for any warm climate crops like tomatoes, peppers, cucumbers or courgettes which need a warm, sheltered spot outside or a polytunnel to thrive.
If you are germinating seeds in a propagator or on a windowsill don’t worry too much about them getting too warm (provided they are kept moist) as the maximum temperature is over 30˚ in most cases but you need to know the minimum if you are sowing without heat. The germination temperatures (1 – min, 2 – opt & 3 – max) in˚C for the most common crops are as follows:
Beetroot 8, 28, 33, Broccoli 7, 25, 30, Brussels Sprouts 6, 3o, 35, Cabbage 10, 30, 35, Carrot 7, 25, 30, Cauliflower 8, 23, 30 Celery 15, 20, 24, Cucumber 20, 30, 35, Kale 10, 32, 37, Leek 8, 25, 30, Lettuce 7, 18, 23, Onion 10, 23, 33 Parsley 10, 23, 30, Parsnip 10, 17, 20, Pea 15, 23, 27, Pepper 18, 30, 37 Tomato 5, 27, 33 Turnip 15, 30, 38.
Take note of lettuce in the list above which has a maximum germination temperature of 23˚. This is a common question for us in Summer when some of you guys think your lettuce seeds are duds if you are germinating in a tunnel or greenhouse.
Daytime Polytunnel temperatures in late Spring and Summer will be way above 23˚ so lettuce simply won’t come up. Place seed trays in a cool shed to overcome this until they sprout, you can then put them back in the tunnel.
Should I sow direct or start seeds in seed trays or modules?
With a couple of exceptions I will cover in a minute I recommend sowing most seeds in modular trays for planting out later, this is especially true early in the season when it is simply too cold to sow outside. Indoor sowing in trays provides a much easier environment for the seeds to germinate and helps them get through the early stages of growth relatively unscathed. I also like the fact that I am more in control at this stage and am able to choose compost, temperature and to protect from frost and pests.
I prefer growing in modular trays rather than open seed trays as the there is less root disturbance when planting out. I think we sometimes underestimate the extent and importance of tiny roots roots which are inevitably broken when a seedling is transplanted. With a modular tray the seedling just pops out as a plug with a neat little root ball ready to plant out; transplant shock is greatly reduced and the plant establishes more quickly.
Modules are trays of cells which are available in different sizes depending on what you are growing and how long they need to spend in the tray before planting out. We use 84 cell modules (pictured above) for most outside crops which are planted out in 4 weeks and larger 6 cell versions for warm climate plants which can be in the modules for 8 weeks or more. If you are concentrating you will have noticed that the 84 cell plants are the ones sown at 18˚while the 6 cell plants are sown at 22˚, simple eh?
The exception to growing in modules are root crops like carrots and parsnips which have long tap roots and will be prone to forking if constricted in any way. Before sowing carrots deep rake the beds with a sturdy metal rake and remove as may stones as is practical. Anything that restricts the baby carrot will cause multiple roots to form called ‘forking’. A heavy clay soil also counts as a restriction so if your soil is sticky and heavy you will struggle with carrots, choose stump rooted varieties or grow in deep pots instead.
Beetroot, radish and turnip are also better sown direct when the soil is warm enough but I have found if they are removed from the modules early (at about 3 weeks) they will do very well. Peas and beans are large, easy to handle seeds that germinate and establish easily so don’t need to be sown in modules but if you want to get a head start they will be quite happy started off indoors.
What is the best compost to use for sowing?
There are two main things to look out for when choosing seed compost, texture and nutrient level. The finer grade a seed compost is the easier it will be for young roots to work their way into it and get established quickly.
The compost should also be free draining to avoid fungal diseases like damping off and to ensure it doesn’t become waterlogged and ‘drown’ the young plants. Remember plant roots need oxygen so there must be air spaces in the compost or the seedling will die. If the compost is too damp the temperature will also be lowered which, as we’ve said, is important for germination.
More plants are killed by over rather than under watering so it is better to keep compost slightly on the dry side that than too wet. There is another advantage to slightly under watering which is that the plant will grow more root to try to find water, this results in a stronger and more extensive root system giving the seedling a better chance of survival when planted out.
Seeds do not need any plant feed in the compost to germinate, all the energy they need is in the seed itself. In fact, high nutrient levels can actually inhibit germination especially if the compost is high in nitrogen. Professional seed composts tend to be low in nutrients which gives a better germination rate (we have found this to be the case) but unless the plants are to be planted out or potted on in 3-4 weeks they will need to be fed with a liquid feed.
You can buy the seed compost we use from us (we use Klasmann organic compost) which is the easiest method but you can also make your own by sieving a low grade multipurpose compost through a soil sieve to remove any lumps. The reason for using a low grade multipurpose compost is it is likely to be low in nutrients. There are plenty of other seed compost recipes using your own garden compost mixed with vermiculite for example but I am out of time, we might go into that in another article.
That’s it for now!
That’s it for now, I need to get some work done. As always, feel free to suggest any topics that you think will be useful, I am always happy to oblige.