Runner Beans originate from a frost-free climate, and hence any degree of frost will almost certainly damage them. It is therefore advisable to sow in the ground no earlier than one week before the last frost is expected. Any seedlings that have appeared and are damaged by an unexpected frost are best removed and new seed sown in their place.
If your seedlings have appeared and there is a chance of there being an overnight frost, giving them some protection by covering them with Cloches will help. You can use plastic bottles with the bottom cut off but with the lid on.
However, the way to be 100 per cent sure that they won't be damaged by frost is to sow indoors and plant out when all risk of frost has gone. A greenhouse isn't necessary for this some space on a windowsill will be fine.
It is unnecessary to soak the seeds before planting as this DOES NOT improve the germination rate, and will in fact cause Halo Blight. Reject any seeds that are not smooth or are disfigured in any way.
Sowing Runner Beans Outdoors
The soil needs to be prepared in advance so that it can settle before planting. We are aiming to provide a soil medium that has plenty of nutrients and is water retentive to some depth. This is achieved by digging to a spade and a half down and adding as much well-rotted organic matter as possible also adding some bonemeal will be beneficial. In the UK this is best done in March.
The seeds should be sown a week before the last frost date (UK Late April). This is done by using a trowel to create a shallow drill (small trench) approx. 2inches / 5cm deep. The seed should be sown thinly approx every 6 inches. If growing in rows the rows should be at least 5 feet apart. Once the seeds have germinated thin out to 1 foot apart and use removed seedlings to fill any gaps this is done 3 weeks after sowing.
Sowing Runner Beans Indoors
Ideally, you will want to use peat pots, these are biodegradable pots, and the seeds that you raise in them can be planted out, pot-and-all, into the garden causing minimal root disturbance and producing stronger, healthier plants. And because the pots lighten as they dry out, you know exactly when to water them! Ensure that the pot is soaking wet at the time of planting out, as this will help the moisture in the soil reach the plant whilst the plant is establishing itself in its new surroundings.
No matter what pots you use the soil needs to be warm and moist and the pots need to be large enough to have 2 inches of compost above the seeds. Fill the pot with compost leaving 2 ½ inches from the top empty, place one seed into the pot and then cover with 2 inches of compost. Water well and place in a frost-free warm place. Do not let the pots dry out.