Gardeners can never really be sure when to start seedlings and when to plant seeds outdoors or indoors. It’s all a case of using a bit of intuition, a bit of historical record and then hedging your bets. However there are some useful things which can help you gauge whether the time is right.
Most seeds and seedlings abhor cold and wet. But received wisdom says that the earlier I plant, the earlier I will get a crop. However I’m not so sure that is true. I can plant lettuces in February, March, and April and so often the later planted ones shoot ahead of the earlier planting. Early planted seedlings so often suffer, unless you give them kid glove treatment. If a seedling is checked by drought, cold or deluge, it may never recover.
In a garden the best time to plant is when the soil is warmed up, when the danger of frost has passed and when the weather looks set fair for sunshine and showers. That is why the peak planting time for most crops is spring and autumn. Warmth, water and light are all required. A dull patch of ground in February, may receive more sun, thus more light, more energy and more warmth a couple of months later.
We can help matters along by warming up the soil with vegetable mulch, with black plastic or by placing a cloches or fleece over prepared ground. Similarly after planting there is a certain amount of nurturing that can be done in open ground. But if it decides to be cold and horrid, your seeds may not germinate.
A late frost can be a killer. Below is the table for AVERAGE last frost dates over Britain. I’m in London, so technically there may still be a frost up to mid May. In fact because I’m right in the middle of London the temperature on my rooftop tends to be about 2 degrees higher that the forecast temperature. I follow temperature avidly. I have little sensors in the beds, in the growhouse and on the north wall, which are linked to a display in the comfort of the lounge. Today my home, without heating is 6 degrees warmer than the air temperature outside, which is 14.5°C. The soil in my raised beds is still cool though, below 10°C. I can increase the soil temperature by protecting delicate seeds with fleece of a cloche. I tend to favour fleece because then rainwater will seep through. But cloches bump up the temperature, especially on a sunny day.
If you are not sure about weather conditions outside then you can control matters better by planting under glass. I’ve found my small scale growhouse excellent all winter, in keeping frosts at bay and for capturing the heat from low winter sun. I automatically water frugally during the cold months, gradually increasing the flow as outside temperatures increase.
I’ve become a fan of regular ventilation. If you start seedlings in too cosy a location they will not enjoy the trip to the great outdoors. Nikki Jabbour, in her excellent book, Year Round Vegetable Gardener recommends opening the vents as soon as the outside temperature gets above 4°C and anywhere above ten degrees should see the vents and doors fully widened. I’m trying this out this spring. One benefit is that crops don’t get overheated under glass and they do seem less prone to mildews and algae.
In the long run there is only really one fool-proof method. That is planting little and often during the spring and autumn rush in the hope that something will come through for you. If you are planting outside, save a few seeds back, they can always be planted inside to fill gaps that appear later.
If I get a weak crop, I have to remind myself that it is unlikely to rally. I can sew again, or I can buy plantlets from the garden centre. Buying in is not a crime in the rooftopvegplot. I might not always get so much choice over varieties, but at least I can fill gaps quickly and attractively.
I shall be planting hardy things like broad beans and lettuces this month, but more tender plants like courgette, beans and peppers will wait until late May or even early June.