I'M HAVING EXACTLY THE SAME PROBLEM WITH MY BASIL!

Did you harden your plants off? Sun and weather can be rather hard for the plants to take if moved directly from the indoors to an outdoor garden bed.

FataMorgana

How much peat? I wonder if your ph is off. I have an indoor herb bed that I use for the winter and I used a lot of peat - which will raise the acidity of the soil some. My herbs all did well except Basil.

Yellow leaves often is a symptom of nitrogen deficiency and if there are nutrients in the soil, but the plant is not taking them in, that is often a ph issue.

While I don't know if ph is the problem or not, I have to add my $0.02 given the discussion about peat & ph...

I don't use peat in my seed starting or potting. I use coir instead. Coir is ph neutral, has more nutrients than peat, and is a by-product of the coconut industry. In larger areas like raised beds, I use compost - which you can buy in great quantities if you can't compost enough yourself. Peat bogs replenish at such an incredibly slow rate that it is not a sustainable resource.

I've attached two pictures I took at a local peat bog. It is a rare and stunning place filled with unusual botanical sights - pitcher plants, sundews, and more. It is threatened by peat harvesting outside of the protected sanctuary. That harvesting is slowly but surely draining and destroying the bog.

FataMorgana

Sundew & Pitcher Plant:

Here is a link that might be useful: Coir info