At a glance
April is the busiest month in the UK seed sowing calendar. Soil temperatures have risen enough across most of the country for direct outdoor sowing of a wide range of crops, indoor-sown seedlings from February and March need pricking out and potting on, tender crops like courgettes, squash and sweetcorn should be started indoors this month, and succession sowing of salads and quick-maturing crops should begin in earnest. Managing this volume of activity without things getting out of hand requires some organisation – it is easy to sow too much at once and find yourself with a glut in June and nothing in July.
Late frosts remain a genuine risk throughout April in most of the UK. The average last frost date is mid-April in the south and late April to early May in the north and at altitude, but these are averages – frosts can and do occur in May even in southern England. This means that tender crops started indoors in April must not be planted outside until after the last frost has passed, however strong and healthy they look. Hardening off – the gradual acclimatisation of indoor-raised plants to outdoor conditions – should begin in late April for plants destined to go out in late May.
Tender crops indoors
Courgettes started in early April will be ready to plant outside in late May – the timing works out almost perfectly. Sow one seed per 9cm pot on its side to prevent water pooling on the flat seed surface, cover with a propagator lid or cling film, and keep at 20°C until germinated. Once the seed leaves are fully open, move to the brightest available position. Courgettes grow rapidly and will need potting on to a larger container within two to three weeks if they cannot go outside. A courgette sitting in too small a pot for too long becomes root-bound and sets its growth back significantly.
Sweetcorn sown in April produces plants that are ready to go out in late May or early June, which is the correct timing for the UK. Sweetcorn must be planted out in a block pattern rather than rows – it is wind-pollinated and planting in rows produces poorly pollinated cobs with gaps. A block of at least 16 plants (4×4) is the minimum for reliable pollination. Varieties bred specifically for UK conditions – Swift, Lark, Incredible – ripen more reliably in a cool British summer than American varieties which need sustained heat to develop fully.
Direct outdoor sowing
Carrots sown in April produce the main summer and autumn harvest. Sow thinly in drills 1cm deep, cover and water, and thin ruthlessly to 5-8cm apart once seedlings are established – overcrowded carrots produce forked, stunted roots. The thinnings smell strongly of carrot as they are removed, which attracts carrot fly. Thinning in the evening and removing the thinnings from the site reduces the risk. Varieties with some carrot fly resistance – Flyaway, Resistafly, Maestro – are worth considering for gardens where carrot fly has been a persistent problem.
Succession sowing
Succession sowing is the single most effective technique for maintaining a continuous harvest through the summer and one that most gardeners understand in theory but fail to apply consistently in practice. The principle is straightforward – instead of sowing a whole packet of lettuce in one go, sow a short row every two weeks. The reward is fresh salad leaves from May through to October rather than a massive glut in June followed by nothing. The same principle applies to radishes, which are ready within four weeks and become woody and unpleasant within a week of reaching maturity, and to peas, where a single large sowing produces a brief but intense harvest followed by empty plants.
Late frosts and protection
Beetroot sown in April produces the main summer crop for harvesting from July onwards. Sow seeds in drills 2.5cm deep and 30cm apart, thinning to 10cm between plants once established. Beetroot seeds are actually clusters of two or three seeds fused together, so each sowing position may produce multiple seedlings – thin to one strong plant per position. Monogerm varieties like Pablo or Moneta have been bred to produce single seeds, which eliminates the need for thinning and is worth specifying if saving time matters. Begin harvesting when roots reach golf-ball size for the most tender texture – large beetroot are edible but become woody and less flavoursome.
April is also the best month to begin establishing a herb garden from seed. Chives, dill, fennel, flat-leaf parsley and coriander all establish well from direct April sowing and will provide pickings from June onwards. Parsley is notoriously slow to germinate – it can take 3-4 weeks and the temptation to resow before the original sowing has had time is common. Mark the row clearly and wait. Pouring boiling water along the drill before sowing is an old allotment technique said to break dormancy and speed germination – the evidence is mixed but it does no harm. Chives are by far the fastest and most productive herb from an April sowing, ready to cut within 6-8 weeks and providing regular harvests for years if the clumps are divided every three years or so.
Sow sweetcorn in April but do not plant it out until the soil temperature reaches 10°C. Sweetcorn planted in cold soil sits sulking for weeks without establishing. Soil at 10°C or above produces strong, rapid root growth and the plants establish quickly. Buy a cheap soil thermometer and use it – it takes 30 seconds and prevents the most common sweetcorn failure in UK gardens, which is planting too early into cold soil and watching nothing happen.
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