As you may know my preference for most plants is to sow indoors in modular trays as you have a much greater chance of success. Freshly germinated seeds sown outdoors can be demolished overnight by slugs just as their first tentative shoots appear.Slugs are often so thorough in their nocturnal grazing that damage is often mistaken for seeds not germinating at all. You can see a row of new seedlings below with their first leaves eaten off by a slug or snail, they are very unlikely to survive. Seeds sown indoors are also protected from the erratic weather at this time of year where frost and cold winds are common.
The other good reason for indoor sowing is you can get more from your garden. You can fairly easily anticipate when a given crop will be finished so sowing some seeds for a follow on crop 4 weeks beforehand means you can get going with a month under your belt when the first crop is finished.
Sowing Methods - Indoor Sowing
We use 84 cell seedling trays which give a seedling plug of about 2 inches in depth. Clearly you won’t need 84 of each plant so plant rows of as many as you need. Make sure to mark the rows with the seed variety and the date you sowed them.
Use a fine seed compost, we use Klasmann professional seed compost with excellent results, I have included it at the bottom of the page. Seed compost is a finer grade and also lower in nutrients than multi purpose compost. High nutrient levels in a multipurpose compost can retard germination with some seeds.
Fill the seed tray with compost and brush off any excess. When filling the tray rub the compost through your hands to break up any lumps. Give the tray a sharp bang on your table to settle. Compress the compost in all the cells with your fingers before repeating the filling process. This is important as if not compacted the compost 'plug' is likely to fall apart when the seedling is removed. It also means the plug is more likely to dry out.
With your fingers make small depressions in each cell about a fingernail or 1.5cm deep. This is an average, some seeds may require a deeper or more shallow hole.
To make sowing small seeds easier, fold a piece of paper to create a tight gutter which makes the seeds line up in a nice neat row. Use a pencil to push each one over the edge as you move along the row. A pencil is much better than a biro as the seeds sometimes stick to the ink at the end of the pen.
Cover the seeds with another layer of compost then scrape across the top of the tray with a stick to remove excess. Don’t compact the compost with your hand, leave it nice and loose making it easy for the seedling to push through.
Gently water your seeds. A good tip is to use a plastic bottle with small holes punched in the cap or the 'bottle top waterer' (pictured below). Be careful with watering as you can do much more damage with too much water than too little in the earlier stages. The compost needs to be moist for germination but not soaking. Don’t forget we have no roots yet to soak up water so the seed can rot in the compost if left too wet.
Outdoor Sowing If sowing outdoors you need to make it as easy as possible for the seed to germinate and to get roots established once it does. Soil needs to be raked to a crumbly even consistency with few stones and without any large lumps, this is what is usually referred to as a 'fine tilth'
Make sure your soil is up to temperature for seed germination, at least 10 degrees C. A soil thermometer will give an accurate reading but to get an idea keep an eye on the weeds in your garden. If they have started to grow again after the Winter the soil is warm enough to sow.
Seed Sowing Depth - As a rule of thumb seeds are sown at a depth approx 2.5 times the size of the seed for large seeds like peas and beans and approx 2cm deep for most other seeds. The exceptions are lettuce, celery and celeriac which need light to germinate so should be sown on the surface with a very light covering of compost or none at all. Seeds sown too deep may fail to germinate (I find carrots can be tricky here) while those sown too shallow can dry out quickly in warm spells when the roots are still too small to gather adequate moisture.
When sowing small seeds like carrots make a drill lined with a mix of soil and compost to give the new roots an easier start, this may not be necessary in your garden but I have quite a heavy clay and have noticed a marked difference using this technique. A drill is simply a little trench hollowed out of the soil at the required sowing depth. Be careful, by the way, when sowing small seed like carrot in drills as the ridge you create can make the drill itself look deeper than it actually is. Scoop a small section of the ridge away to get a better indication of the depth of trench in relation to the surrounding soil. You can see this mentioned, among other fascinating facts, by clicking the play button on the video above.