Those are not rules. They are based on the length of time it should take to reach the size needed for transplanting. A Tomato or pepper should be at least close to 6 inches before setting out and have a good root system. The latter is the most important. Small seedlings just don't have a good root system and are more fragile. They are easily killed by adverse weather conditions.

In addition your soil must be warm. If you can't dig under the first layer of soil and feel the warmth then it's too cool for Tomatoes and peppers

The soil is definitely warm. It's been in the 90's. I have watermelons and cucumbers among other warm weather crops growing that were direct seeded. I have a group of tomato starts that are much taller than the rest and thought they may be tall enough to go out, but they aren't quite 6 inches yet.

They are your plants so of course you can set them out whenever you wish as long as you understand the risks, that the odds are not good for them, and you are willing to accept the results - be they good or bad. There won't be any "plant police" who come along and arrest you for plant abuse. ;)

I wouldn't do it but some folks will transplant both tomatoes and peppers at the cotyledon stage. They are willing to accept a 50% loss of plants, even under ideal conditions, in exchange for the jump start some of the survivors get.

It isn't a common practice by any means but why not try just a few of yours while holding some back in reserve and see how it works for you in your garden. Just keep good notes on all the details so next year you'll know when to set them out and when NOT to.

Dave

So I guess what I am asking, is how do you know when they are ready? It's been a month; they are between 4 and 5 inches tall; the roots are starting to poke through; they have true leaves; and the weather is warm enough. (I already have a few transplants out that I bought at the store and they are doing well, but they were a bit taller than mine)

Is it better to wait till they are 6 inches and keep them under lights or put them outside in the warm sunshine now even though they aren't too tall?

Well since they were raised inside they will have to be hardened off first anyway before they can be transplanted. Nothing can go from the inside directly to the garden. ;)

Hardening off takes about a week since it is a very gradual process so a week from now when they are fully hardened off (lots of posts here on how to do that or just Google 'harden off plants') they will be a bit bigger.

So I guess what I am asking, is how do you know when they are ready?

This is for future reference.

(1) are they still in their original container or have they been transplanted at least once? If not, then they aren't ready. The ones you bought had already been transplanted 2x.

(2) Do they have at least 2 full sets of true leaves? If not, they aren't ready since they don't have enough top growth OR root structure to keep them going. This is where the 6-8" rule comes in as that is the average height of a plant ready to go.

(3) Are they at least 6 weeks old? If not, then they probably haven't developed a sufficient root structure yet. ESPECIALLY if they have never been transplanted from the original container.

(4) Peppers are very seldom transplanted to the garden at the same time as peppers. There is usually at least a 2 week gap between since peppers require much warmer soil temps than tomatoes. So measure your soil temperatures. Don't just assume it is warm enough. Air temp does not = soil temps 6" down.

But none of this is cast in stone. They are "guidelines" developed to help insure the best chance of success. But as I said, you have to experiment a bit to see what works best for you.

Bottom line is that for your zone you started them a bit too late - note that for next year and back up 2 weeks. So given that, your patience now, if you can hold out for another week to 10 days, will be rewarded.

Hope this helps.

Dave

Thank you. That helped a lot!

Another reason to hold back would be if your last average frost date is still in the future. Just because the weather's warm now, does not mean the warmth is here to stay. We had temps in the 80's at the beginning of the week. Yesterday the high was 45 and the hilltowns had night temps in the twenties. Planting out early would have been disaster, unless the gardener had a way to protect the plants from a hard freeze. You're in a warmer zone, so you may be good to go.

Those guidelines for what to start when are intended to help the gardener have a healthy plant of an appropriate size at the safe plant-out date, usually the last average frost date.