good afternoon, what i have read/been told is that you should wait till the roots have fully colonized the current medium it is in(when you pull the root ball out all the soil should be held together unless in the 5-1-1 gritty mix) you can find LOTS of great info on the site if you use the search bar, and I am sure someone with a wealth of knowledge will chime in and give you the answer you are looking for. this is what i do whith my plants.

Peppers grow much slower than maters. I usually put my peppers in the ground when the day temps avg between 70 and 90 and the nights no colder than 50 degrees celcius . As long as you do not destroy the roots you will be fine Like spongey said. I have bhuts jolakias I started in january that are barely 4 inches tall and maters i started in March 5 feet tall. I have noticed the hotter the pepper type the slower and harder to grow. Just my experience.

I dont think it matters much how big the plant is when ya plant it out. I think temperature is more an issue. Romy was talking about nightime temps of 50 Celsius, i think that is no good for peppers.

I would go with planting them out when the nightime temps are steadily above 50 FAHRENHEIT. I believe root development slows down around 50 and stalls out when colder. And if the roots aint functioning yer plants wont grow and thrive.

Jim

all my peppers right now are between 4-7" tall. they are growing slower due to the colder than usual weather. like the past 2 weeks we have been sub 70 degrees, but our nights are above 50 so i am keeping the peppers inside for another couple weeks to harden them off and then they will be outside and hopwfully grow a little! LOL

I think those are fine to plant outside. If you get them from a garden store, they probably wont be much bigger than that. Just make sure they are hardened off before you kick them out of the house.
Bruce

I agree with the others.

Small transplants are fine as long as they have been hardened off and have sufficient root-mass.
I grow in the Gritty Mix and the 5-1-1 and the roots will hold the mix together when fully colonized.
This year, I made the mistake of leaving my super hots in small containers for too long, and their
growth was severely curtailed. Once I moved them into 4-inch containers, the growth resumed.

Cold soils and night-time temps below 50F will slow growth and lead to yellowing plants.

Josh