I like to plant my seedlings about 30 days after sowing, that regular 30 day tempo makes planning so much easier and it means that the plants are young and vigorous and haven't run out of nutrients in their compost.
<aside> 💡 If you use growlights you will probably find plants are ready after 21 days, rather than 30
Unfortunately this isn't always possible, some plants just take longer than this to mature to planting size and that's particularly true of the fruits. Cucumbers, tomatoes, peppers, summer and winter squash, which generally take 6-10 weeks. It's also generally true of the onions and brassicas, which often take 2 months. Over-wintered brassicas take the record for life in pots though as they can be sown in September/October and not planted out until February, but then I have plenty of space at that time of year.
Weather conditions are of course the main criteria for deciding when to plant. Taking a look at the long range 10 day forecast is ideal if you are planting something that will suffer from cold-weather, high winds or blistering heat.
We grow a lot of our early and late crops under cover in cold-frames, low tunnels and a big polytunnel, so we are able to plant at almost any-time without regard to the weather.
if you don't have any way to protect your small plants, then covering with fleece has a similar benefits, but you might need to lift the fleece off the plants if wind or heavy rain threatens.
In hot weather I will sometimes use shade cloth over my plants for the first week or so, until they get established.
Although I hardly ever harden off my plants, it is a good practice. Move plants from their protected growing environment into a sheltered spot outdoors for an hour a day, then two hours, then three etc. Gradually extend the time and level of exposure. A similar approach can be achieved by placing plants in a cold-frame to harden off and gradually opening the lid wider each day. For more on hardening off I have a chapter that goes into the details.
You need to consider six things when deciding to to plant:
Let's dig into these a bit more: