Germination is the process by which an organism grows from a seed or similar structure and develops into a new plant. Three key environmental factors are important to trigger the seed to grow. We are going to call these the Germination Triangle and these factors are :
- how how much water is available
- the temperature
- the planting depth of the seed
A careful balance is needed between these three factors. How dependent the seed is on them varies depending on the plant. Some seeds will grow anywhere (and we wish they wouldn't!) whilst some need infuriatingly perfect conditions to germinate.
Water is key as the first stage is for the seed to fill with water in a process called imbibition. The water activates special proteins, called enzymes which that begin the process of seed growth.
First the seed uses the carbohydrates and proteins stored inside to grows a root (radicle). The root accesses water before the next stage begins: sending up a shoot above ground. As the shoot develops there will be secondary root formation and branching of the roots.
By now the seed's reserves are running out so the next stage is to grow leaves to harvest energy from the sun. The leaves continue to grow towards the light source in a process called photomorphogenesis.
Light is very important at this stage. If there isn't enough light this causes the plants to become etiolated. This is a natural adaptation to help the shoots elongate quickly to break through the soil and reach the light. However, if it takes too long to reach light the resulting plants will not be strong. The seedlings will become elongated, spindly, leafless and pale with a poor root system.
The Germination Triangle
The amount of water has to be just right for optimum growth. Too little and the seed won't grow. Too much and the seed will be unable to access the oxygen in the soil and won't develop. It will basically drown.
With careful watering, this balance is simple to achieve when you germinate your seeds in seed trays or pots. If you are sowing direct outside preparing the soil ahead of planting will help you get the water balance right. Ways you could do this include:-
- Keep off the soil to prevent compacting - if you have to walk across it then lay long planks to use
- Aerate the soil - if there are no air gaps then you can create them by aerating the soil with a garden fork or machines that do the job can be hired easily
- Dig through a balanced fertiliser to break up and improve the soil
- Use a Raised Bed - the easy way to do it but remember not to walk on it and compact the soil
- If your plot is very waterlogged adding a ditch or seasonal pond at the lowest part of the garden for excess water to soak away will be helpful. This has the advantage of creating a habitat for slug eating frogs and toads which you can read more about in our Pests & Diseases - Slugs & snails blog.
Temperature is also an important factor. The temperature a seed needs to germinate will often be determined by where the plant originates. Those that come from Northern climates will often germinate at cooler temperatures than those native to the tropics. Of course there are exceptions to any rule but many seeds will only germinate when the weather reaches spring temperatures. This can lead to confusion in plants when a freak warm spell causes them to germinate too early leaving them vulnerable to frosts which should be over.
Some seeds only germinate after extreme temperatures, such as after a forest fire or an extended cold period but there aren't many of these plants in your average veg garden.
The best way to judge soil temperature is to test with a metal thermometer. Insert it 3 or 4 inches into the soil 3 or 4 days in a row. The soil can be warmed by the sun later in the day so morning is the best time to test. If the soil isn't warm enough then warming it with a Seedling Tunnel is a good option to allow you to start planting earlier than your neighbours.
Charts are available online to tell you what temperature a particular plant may prefer. From an optimum 40 degrees for a pea plant to 70 degrees for a tomato. Once you feel the soil is warm enough, check the long range weather forecast for expected cold snaps and you should be safe to plant. If the forecast fails you then methods such as or using Bell Cloches or Poly tunnels to shield young seedlings are foremost in the gardener's armoury to start successful germination.
Planting at the right depth improves the seeds chances dramatically and will increase your germination success rate. The seed will only store enough energy to sprout and reach the light so planting too deep may mean it does not have the energy to make it out of the ground. If you plant it too shallow then it may fall prey to birds or dehydrate preventing germination.
Generally the seed packet will clearly show the planting depth so is simple enough to achieve. But what if a friend gave you the seeds, you harvested them yourself or just lost the packet? You can look it up on the seed company's website or check out similar packets at your garden centre. Or you could try and work it out yourself.
To calculate it yourself, the word on the allotment is that seeds should be planted no deeper than two (or three - opinions vary!) times their diameter. This may differ from what's on the seed packet which too often seems to be 1/4 of an inch. So experimenting planting some at the packet depth and some at the calculated depth might improve germination rates.
Some seeds actually need light to germinate e.g. lettuce and dill. These tend to be very tiny and should be placed on the surface of the soil and not covered. The challenge with these is to keep them moist as they quickly dehydrate without a covering. If growing in seed trays - cover with plastic to prevent the water evaporating away. Or cover with a fine layer of vermiculite - a soilless, mineral growing medium - which is porous and lets light shine through, while keeping enough water around the seeds so that they remain moist.
Those that need to be planted deeper can benefit from Rootrainers where deeper cells can be chosen for plants that need the extra depth and moisture can be retained by using the integral cover.
The final piece in the puzzle is where you plant them. Some seedlings such as carrots do not like to be transplanted so it is important to sow these in the position you want them to grow. Others can handle being moved so give you an opportunity to make the most of the short growing season by starting them off inside and transplanting them into growing position the minute growing conditions are right.
I hope that this has given you some tips to increase the germination success rate in your garden. Given the delicacy of the balance that needs to be struck for healthy robust seedlings to germinate and grow you can only marvel at how the self seeders in your garden manage so easily what gardeners work so hard to achieve.