One of the newest arrivals in the windowbox is a plant of what is almost certainly smooth tare, a mini-vetch. It appeared in the autumn and has been growing steadily since.
For the last few days the weather has been very cold and frosty and the soil in the windowbox will, by now, be frozen right through. This makes it a much harsher microclimate than the garden round about where the roots of many plants will be below the level that frost can penetrate.
We will see what survives (the previous two winters have not been as cold as this).