How to get strong roots when starting seeds indoors.

Last year I started several seeds indoors. I have a large bay area in my home with full sun virtually all day, even in the winter, so lights aren't really required. Last year was my first year starting seeds indoors and although the above the ground growth was good, there wasn't a whole lot going on underneath....
I am looking for any suggestions you may have to build stronger roots!!!

What are you planting the seeds in? What kind of soil? What kinds of plants? Need details.

I'm starting a whole bunch of everything; veggies, perennials, some annuals. They are all started in a soil-less pro-mix which I was told is the best. The ones that transplant well are in cell packs and the rest are in jiffy pots. They are covered with plastic covers, so the moisture level is pretty good. I have also started most of them with a seedling heat pad underneath.

Someone told me to take willow branches, cut them up and soak them and then water the seedlings with that water. Apparently it's supposed to creatate more/stronger roots. I'm just not sure if that's the way to go.

Although you have a large bay window with sun all day, it isn't enough light. Plants started from seed in the house need a minimum of 16 hours of light every day. Only plant lights can supply that.

What makes you think the plant don't have strong roots?

You said that top growth is good, but not the roots. Are you sure that the plants aren't leggy from stretching for light. If the space between leaves is long, then they aren't getting enough light. There is alway a balance between top growth and root growth in plants. Plants have to develop enough roots first before they can support top growth and develop leaves.

As far as the willow branch water, I have never heard of that. Must be an old wife's tale. Roots will grow when the plant receive what it needs. The soil you used is fine. However, after 6 weeks the nutrients incorporated in the mix will be used up. Use Miracle-Gro, or similar, and dilute 1/4 strengt while young seedlings. You don't want it strong at this point because you can burn the roots.

The willow branch thing is for cuttings not for seedlings--willows contain a chemical that is similar to the chemicals in rooting hormone so supposedly willow water can help get cuttings to root. I've personally never tried it so I have no idea how effective it is compared to the commercially available rooting hormones, but chemically it's similar so it could have some effect. I'm not sure if rooting hormone does much for plants that already have roots though, their primary function is to help things make roots when they don't have any.

Willow makes sense to me they form roots so fast and strong and no matter what you do as long as they can find water..they grow. So it is probably the best free tip we will get this day. It must be comparable to compost teas.

I'll stick to my rooting hormone