After the slower months at the beginning of the year, April is when things start to get exciting and even overwhelming in the garden and greenhouse! It can be tempting to sow everything, because the seedlings spring up so quickly, but beware of the risk of frost - although they tend to be light, plants can still be set back significantly by a cold snap.
You can start to harden off hardy annuals now - this involves giving your plants more and more exposure to weather. I used to think that the most important factor in hardening off plants was getting them used to the cold. However, the wind and direct sun can also overwhelm small seedlings. So, put your plants in a sheltered position, perhaps near a wall or under a shelter, and consider moving them back inside for part of the day (or night, if it’s cold) to begin with. The key is doing everything slowly.
In April, you can start to sow the half-hardy annuals. These include zinnias, cosmos and nicotiana. Many of these germinate best with some warmth, so consider investing in one. You will need to keep your greenhouse frost-free but at this time of year that is easily manageable with some horticultural if the weather takes a nosedive.
My favourite job in April is to pot up dahlias. This gets some plants started early to ensure the earliest possible blooms from them. Dahlias shoots are not at all frost hardy, so only do this if you have room to keep them inside. Mine are in my greenhouse. Each tuber will need its own pot and rich compost. Water well once and then leave alone until you see the first shoots emerge from the compost. Don’t forget to label the pots so you know who is who when it comes to planting.