With Late Frosts Expected In The UK, Protecting Young Seedlings Is Vital. The Safest April Approach Is To Start Seeds In Pots Or Modules, Even Hardy Ones, So They Can Be Moved Under Cover If Frost Threatens. However, Many Vegetable Crops Can Still Be Sown Outdoors This Month.
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Of course the usual proviso about location still applies. If you are further north, at higher altitudes or in a frost pocket expect to sow later or protect longer. Part of the secret however lies in knowing what will happen survive the frost and what definitely needs protection. Warmth lovers such tomatoes, cucumbers, melons, peppers and chilli love it warm. Ideally 10C minimum though they will cope with lower for brief periods on some occasions. However, any cold shock will set them back and will delay flowering and fruiting. If you are OK with that then that is fine. But a later sowing might have been easier.
Here are my suggestions for you.
Sow Outdoors (Hardy & Semi-Hardy)
These seeds can be sown directly into prepared soil outside now. They are resilient to cold soil and light frosts.
Carrots: Sow directly in a sunny, weed-free spot.
Beetroot: An easy crop for beginners; sow direct for a summer harvest.
Parsnips: Sow seeds directly where they are to grow.
Peas: Both early and maincrop varieties can be sown directly.
Broad Beans: Make a final sowing now for a later harvest.
Spinach: Thrives in the cooler conditions of April.
Lettuce: Sow directly, but protect seedlings if a hard frost is forecast.
Radishes: Quick to germinate and great for filling gaps.
Spring Onions: A reliable and easy-to-grow crop for direct sowing.
Onion & Shallot Sets: Plant these directly into the ground.
Potatoes: Plant early and second early varieties, earthing up shoots to protect from late frosts.
Beetroot are not overly frost sensitive and wil cope with all but the most severe frosts.
All of the above except carrots and parsnips can be started in modules and transplanted if you wish. Carrots and parsnips are sensitive to being transplanted and often suffer root damage if this is attempted. Even potatoes can be started in small pots and transplanted .. though few people bother to do so as it is time consuming.
Sow Under Cover (Tender)
These seeds need warmth to germinate. Start them indoors, in a greenhouse, or on a sunny windowsill. Do not plant them outside until the risk of frost has passed, usually in late May or early June.
Tomatoes: Sow indoors on a warm windowsill or in a propagator.
Courgettes: Start under cover from mid-to-late April for planting out later.
Cucumbers: Need warmth; start indoors in April.
Squash & Pumpkins: Start in pots under cover; they are very sensitive to cold.
French & Runner Beans: Start indoors in pots in late April to get a head start in mild areas. But beware going too early. Every years I see photos of beans that are 5-6 high with questions about when they can be planted out!
Sweetcorn: Can be sown individually in modules under cover; it hates root disturbance. But I would much prefer to sow in the soil in May. They catch up and are much better plants.
Basil: This tender herb is best kept indoors until May.
How to Provide Frost Protection .. and keep the cold winds off
If you choose to sow any seeds directly outdoors, be prepared to act on frosty nights. But only for the more tender ones. Many cope very well with a frost. Though you might want to cover some seedlings with a fleece to protect from pests. Below are your options
Use Cloches: Protect individual plants or rows with glass or plastic cloches.
Horticultural Fleece: A lightweight fabric that can be laid directly over plants. Secure the edges with stones or soil.
DIY Covers: Old compost bags or plastic sheeting can be used to cover seeded areas overnight.
Cold Frames: Ideal for “hardening off” young plants before they are planted out. Though se my separate article on how professionals harden off plants
Additional Considerations for April Sowing
Check Soil Temperature: Seeds won’t germinate in cold, wet soil. A good rule of thumb is that if the soil feels too cold for your hand, it’s too cold for most seeds.
Succession Sowing: Instead of sowing a whole row of lettuce or radishes at once, sow a small amount every two weeks. This will give you a continuous harvest rather than a sudden glut.
