In my way of thinking it's never too early to root runners as you need well-established runners before the winter if you are going to get a worthwhile crop the first year. The second year is the heaviest yield and if you can manage with just these two crops you will get the best quality of fruit and probably keep the stock more vigorous. If you have a runner that already has a root system when you find it you stand a good chance of rooting it independently, rather more as if it is a cutting.
I tried to root the runners too late last year and lost a lot in winter from pests and diseases as a consequence.
Allan
