“Stroking – gently brushing or stroking seedlings raised indoors helps to produce stocky, firmly rooted plants”
“Seedlings raised under cover tend to be taller and weaker than those growing outside. Gently stroking or brushing seedlings makes them grow shorter and sturdier without harming them in any way. The plants do not need to be stroked individually. A piece of card brushed gently backwards and forwards 10-20 times over a tray of seedlings is all that is required on a daily basis. Plants seem to respond best when stroked early in the day.”
Quoted from Organic Gardening in Australia