For the last couple of years, we’ve grown so used to unpredictable weather that we now sow nearly all of our seeds indoors. But, for those crops that need to be directly sown in the garden, like turnips, potatoes, radish and lettuce, there are a few tricks to improve germination rates outdoors.
In this article we’re going to explore how to warm your soil, find the best position for seedlings, and just what gardeners mean when we talk about vernalisation and scarification.
What is seed germination?
Seed germination is the art of recreating natural environments to encourage seeds, and in some cases, tubers, to sprout into new plants. In the vegetable garden, that makes seed germination the most important part of the gardening year. Where we often go wrong as gardeners though, is to assume that seed germination is the first act of the gardening year.
In this article we’re going to run through some of the most important things that all gardeners skip, avoid, or just forget altogether, which can improve seed germination outdoors. All of these jobs should come before any seed touches the soil at all!
How to improve seed germination outside
Of all the tricks to improve seed germination outside in this article there are three golden rules that should always be observed when preparing and planning your garden in spring:
- Warm up your soil
- Match seeds to soil nutrients
- Sow at the right time
Finding the best location to sow seeds outdoors
Finding the right location is important to improve seed germination. Most seeds need as much sun as possible and good drainage. Both factors help to reduce the chances of funguses or bacteria killing seedlings before they sprout, or calcium overwhelming young seeds in the soil.
Use walls for heat and protection
If you have a brick, concrete or rendered wall in the garden, this is a great place to germinate seedlings as they will benefit from the wall as a wind barrier, as well as a heat source. Creating a raised bed next to a solid wall, packed with nutritious compost and free-draining topsoil can provide a perfect germination spot.
Think differently about greenhouses
If you have a greenhouse, or cold frame, the more controlled temperatures can also help germinate seeds more reliably outdoors. The warm late springs of late have been enough to frazzle many of our seedlings in the greenhouse, so we now keep staging just outside the greenhouse. Here, the evening heat lost from the greenhouse seems to keep them warm enough, without burning them as it would if they were under the glass.
Preparing soil to improve seed germination
You might hear gardeners talk about the tilth of their soil. All this means is the texture and density of their soil surface. Raised beds that are regularly cultivated and raked in spring will typically have a fine tilth, meaning they are free from clumps and stones, and the soil has a fine and even texture.
If you sow seeds into chunky, poorly drained soil, or raised beds full of stones or clay, your seedlings will be shaded, damp, and grow unevenly. That should be job number one in spring when you’re trying to improve seed germination outdoors.
Understanding when soil is ready to sow
Once you’ve prepared your soil, cleared it of weeds, and added nutrients back through, it’s important to make sure the soil is actually warm enough to germinate anything. There are a few factors that impact soil temperature:
For much of Europe and North America, cold winters can leave residual frosts in the ground, and soil beneath the surface can take several weeks to fully thaw before it’s ready to plant into. Wet winters and shorter days in spring for more Northern climates keep temperatures down.
How to warm up soil to improve seed germination outdoors
To improve germination rates outdoors, try warming your soil before planting into it. Cloches made from fleece, plastic or polythene are incredibly effective at creating microclimates and thawing soils after cold winters.
For cold climates in particular this can often give you a two-week head start on planting seedlings and plastic cloches can prevent frosts from ever building in the first place if you use them to protect and warm your soil through winter, and into spring.
To build a basic cloche for heating soil,
You will need:
- Treated timber or bendable steel bar
- Polythene sheet, corrugated plastic, Perspex or fleece
- Note: corrugated plastic is the best material as it bends easily around curves, which heats the soil more evenly
- Screws / Cable ties
- Create two ends for your cloche (either triangles from timber, or semi-circles from steel bar)
- Attach your chosen cloches material
- For hard plastics, drill the edges and then screw them securely into timber ends. Make sure the cloche is the right length for your raised bed.
- For steel bars, use fleece or polythene and cable tie it onto each end of the cloche, having pushed the bar into the ground.
Both methods, no matter how make-shift they may look, will protect your soil from the worst of the frosts and speed up thawing in spring.
Tricks to improve seed germination rates outdoors
Seeds germinated outdoors often need some trickery to help them germinate, and different seeds will respond better to different tricks. Soaking seeds might sound like a great way to get seeds started, but you might be surprised that it can actually damage many seeds’ chances of germination instead of help. The other method is called stratification.
Stratification can improve germination
Stratification is the act of tricking seeds into germination by subjecting them to the physical conditions they would experience in nature before germination. For vegetable seeds there are two types of stratification that are commonly used:
- Vernalisation (chilling)
- Scarification (scratching)
Vernalisation can improve seed germination
Vernalisation involves chilling seeds and works well for biennial vegetables like beetroot, brassicas and alliums in particular. While not all these vegetables are grown as biennial crops in our gardens due to freezing winters, they germinate better after one or two weeks in the fridge.
- Place seeds in an airtight plastic bag with a dry sheet of paper towel
- Secure the bag, and place it at the back of your refrigerator
- Leave the bag there for 1-2 weeks (check occasionally for moisture and change the container if they are at all wet)
- After 1-2 weeks, thaw seeds for 1-2 days
- Sow directly into raised beds in spring
What seeds should be vernalised?
- Brussel Sprouts
- Collard Greens
Note: Stratification also refers to heating or warming seeds, but there are no vegetable seeds that will benefit from this treatment before planting. The best way to warm seeds is by pre-warming the soil using the method described earlier in the article.
Does scarification improve seed germination?
Scarification helps to germinate seeds faster, but not necessarily better. Scarifying seeds means scratching their surfaces with coarse sandpaper to break down tough outer shells, and was traditionally done to speed up the germination of beans, squash and solanums, like tomatoes or peppers.
What we know now is that when sowing these plants away from their tropical habitats they tend to suffer from fungal problems and damp off much more readily if their surfaces are scarified before sowing.
So yes, scarification does improve germination rates outdoors but also increases the chance of problems later.
Soaking seeds to improve germination
Large seeds like beans and squashes do benefit from soaking before planting. Soaking seeds helps to gently soften their outer shells and plumps up dried seeds which have been stored over winter. Soaking seeds both improves germination rates outdoors and speeds up outdoor germination.
There are some perennial seeds that benefit from soaking for 2-3 days, but vegetable seeds should only be soaked overnight, or for a maximum of 24 hours before planting. Oversoaking vegetable seeds can spoil them and cause them to split their shells before they are ready to plant.
What seeds should be soaked?
Cover your seeds and seedlings
Finally, when direct sowing seeds outdoors, the most reliable method of improving germination has always, and will always be… cover your seedlings. Covering seedlings improves germination, continues warming the soil and provides regular gentle humidity to developing plants.
Not only does covering improve the conditions for seed germination outdoors but, it can also protect against pests as seeds start to sprout.
Mesh, plastic, glass or fabric – what’s best to improve seed germination?
There are three ways to cover and protect seedlings from pests and frosts:
- Plastic / Glass
- Fabric / Fleece
Choosing the right method depends almost entirely on what your main priority is.
For example, we have a wildlife pond meaning we’ve got plenty of frogs to eat our slugs and, other than pigeons, don’t often suffer from pest damage to our seedlings. So our priority is heat. The warmer we can keep our outdoor seedlings, the better.
Mesh covers are great for keeping birds and moths away from seedlings. For brassicas, mesh is a great choice as it prevents early season cabbage white butterflies from landing on young kale and cabbages. The downside of mesh covers for your crops is that they don’t do anything to warm the soil, so germination will remain the same.
Plastic or glass covers for seeds are great, particularly if you already have cloches for warming the soil over winter. They will continue warming seeds, and the soil around them, and give a good layer of protection from common pests. The downside of cloche covers is that they should always be open-ended to provide ventilation so will not stop aphids, carrot flies or moths from getting to your crops when seeds germinate.
Fleece covers for seeds sown directly outdoor are useful for climates that aren’t that cold but want to vernalise seeds naturally. Beetroot and alliums in particularly can work very well when direct-sown outdoors under fleece through winter. A light frost will help them to germinate in spring. The downside of this is that they can often germinate too soon.
For more information on how to sow seeds, as well as how to improve germinate rates outdoors, read our full guide on direct sowing seeds.
Climate change has shifted how we garden, so the more we can do to improve germinate rates outdoors, the better. As winters stay warmer, but springs stay cool and mild, germinating seeds outdoors is getting harder and harder. Hopefully, some of the tricks above work as well for you as they have for us in recent years.