Seeds : How do you know when it's safe to sow?

Mixed seedlings ready to harden off 

You've made your lists, drawn out your crop rotation plans, have your seeds and compost ready, and before long you'll be itching to start sowing.

The pressure builds as magazines and social media start to fill with stories of planting dates and there's a noticeable jostle over who's starting to sow which vegetable seeds when.

Does that sound familiar? If you're new to vegetable gardening beware as it soon will be and before you know it you'll be sowing too and then wondering why your seedlings are seriously struggling or have died.

Leggy parsley seedlings sown too early and stretching for light

As such an avid social media user, last year I remember feeling a sense of panic that I'd left my sowing too late, yet it wasn't even March! I know from experience that we can be three weeks behind the growing conditions of warmer gardens, but it was difficult not to feel left behind when being bombarded with everyone else's reports.

Don't get caught out by a late frost - frozen seedlings!

So why is it important to sow seeds at the correct time (temperature)?
If you attempt to sow seeds below their preferred temperature, it can result in slow germination, seeds rotting, problems with plants developing and ultimately disease as they wont have started life with the best start.

Seeds need heat to germinate and the requirements vary from plant to plant. They will germinate quicker at higher temperatures but the minimums required can be roughly divided into three groups:

5 deg C for brassicas (cabbage type family), lettuce, peas and broad beans
7 deg C  for parsnips, carrots, beetroot, scallions, leeks, onions
10-12 deg C for runner beans, French beans, courgettes, tomatoes and sweet corn

Different containers for seeds

So how do you know when is the best time to sow?

Part of being a successful gardener is learning to work with, understand and appreciate nature and her elements. That means noticing the wind directions and the signs of seasons changing, the fluctuations in temperature. Keeping a diary of not only what and when you sow but weather conditions that can be referred back to will become an invaluable aid.

If you wish to sow seeds indoors that can be transplanted out as the temperatures rise, or plant seeds directly into the soil where they will germinate, you'll need to have an idea of the soil temperature.

We can roughly work out when the best sowing dates are using average temperatures from previous years. Having an idea when the last frost date is in your area will help you to plan when to sow your seeds indoors, ready to be transplanted outside and hardened off once the soil temperature has warmed up.

So many things wrong here,   too many seeds sown, not enough light, no drainage...

Take runner beans for instance. You have two choices:

1. Sow the beans directly into the soil once the soil temperatures have reached 10-12 deg C for three days or more (between May and June, depending upon where you lives) - NOTE: easiest method!

So for us at 1,000 m above sea level I would be aiming for early to mid June however, if you live in a warm, sheltered area, mid May should work for you.

2. Sow the beans in modules ready to plant out when soil temps are 10-12 deg C or more. 

Beans take between 7 - 14 days to germinate and you could allow approx 3-4 weeks in the pots prior to transplanting outside. As an example, if I'm looking at the calender, aiming to plant my beans outside on 11th June, I would plan to plant them in pots during mid to end of April (keeping an eye out on long range weather forecasts. This should also allow time to harden them off before planting i.e. acclimatising them to the outdoors gradually.

There's no science here - most vegetable guides will suggest you sow runner beans between April and May.  The above was just a demonstration of how the guides arrive at that.

Bottom heat from a propagator gives stronger seedlings

In Ireland the most helpful resource I've found for frost dates, etc are the reports published by Met Eirann that give 30 year average temperatures from 1961 to 1990. Unfortunately not up-to-date, these can nevertheless be used as a guide for the average number of (ground) frost days that occurred each month as well as minimum and maximum temperatures. Bare in mind that the weather conditions from your nearest weather station might not resemble those of your garden. In our case it can differ by up to 5 deg C

Children love to help sow seeds

For more accurate current readings Met Eirann also publish daily records, those combined with the averages could give you a very clear indication of the ultimate sowing dates.

If you want to be totally sure however, soil thermometers are readily available either online or from garden centres.

Horticultural fleece placed over beds will protect seedlings from frost

Whenever you choose to sow, it's a good idea to start gathering resources in case of unexpected frosts that can either be used at the beginning of the year to protect sowings and seedlings, or at the end to protect from autumn frosts. Start stocking up on horticultural fleece, collect newspapers or pick up old net curtains from boot sales so that you can cover precious seedlings should a cold snap occur.

Hope that helps!