Here is a great hint from Dazzarelli, all done with mirrors…
I saw your post on leggy pea seedlings, including the photo from Eveline Fleury, and thought I’d let you know how I’ve got round the problem. I’ve emailed you so that I could attach some photos.
My greenhouse is against a South-facing fence and behind the fence is the next door house. I found that seedlings were leaning away from the dark fence and towards the light. Turning them frequently didn’t help much as the seedlings still ended up leggy. I bought a cheap mirror from Ikea and fixed it to the fence side of the greenhouse and this has completely fixed the leaning and leggy seedling problem. Encouraged by this, I started using aluminium foil behind seedlings on window sills and found that if I used enough foil, and positioned it well, then the seedling grew sturdy and straight. I’ve even had some seedlings, usually the ones furthest from the window, bending slightly towards the foil.
I’ve attached a photo of the mirror in the greenhouse. If you look carefully then you can see the fence at the top of the picture. Most of the picture is taken up by the mirror which is covering the fence. Also attached is a picture taken from outside a window looking in at my Tomato seedlings with the aluminium foil reflector.