How do keep my seedlings from becoming stringy?

I had 12 seeds ,scarified them in a match box,soaked in water 24hrs,set them in a paper towel,put them in rock wool,with heating mat Only 3 grew and they were stringy had to pull them .i have more seeds.what can i do differently to stop the stringy and get more to sprout? plz & thank you.

Take your seeds and put them into a shot glass full of water. Cover that with a coffee cup and place it in a dark closet. Preferable temperature should be at least 70F-90F. Leave those to soak for 24hours or until the first seed cracks. I usually start at night and check in the morning most time they've opened up.

Take the seeds and place them into your rockwool cube with the pointed end up between 1/4-1/2" deep. The bottom of the seed is the larger round end with the hollow indention. If the seed has cracked your wanting the opening up, where the root tip emerges. If you have the cubes with a round center missing you may want to use a jiffy pellet or rapid rooter first.

NowwWater the cube down good one time and put it under a light. Try to keep the light close so the top of the cube stays warm. You shouldn't need to water again until the seed comes up or the cube is losing most of its weight. Your sprouts should be up in 2-3 days.

When they're sprouting and showing additional leaves your best stringy stem cure is adequate airflow to simulate "wind". In nature this strengthens the stem.

Another thing you can do if they get tall and lanky is to transplant them into a plastic beer cup planting them deep. You can the bury about half of the stem in the dirt filling it up near the rim so you have a nice short plant. Now keep the fan on them and get the light right on top of them to keep them from stretching more.