Here in the states, OHSA requires that planks that you will be standing on (like scaffolding) be free of knots. It's probably overkill, but if you need to stand on them in the future, I'd suggest that you set the planks on blocks out in the garden and make sure that they can support you safely. That way if they break, you'll fall a foot to the grass instead of lots of feet to the nice hard stairs.
