I have seed containers and want to plant seeds for this year's garden
Here's an article that should help:
Can you use seeds from store bought veggies for garden
I actually knew of someone who planted tomato seeds from a British Rail cheese and tomato sandwich and they grew.
Just wondering how old is TOO OLD? I have seeds that are more than 19-yrs old. Kept in a box (hence dry & dark place) but I wonder if they will still grow?
This number varies from plant to plant. Some seeds, such as Colocasia, will fail to germinate within minutes of drying out completely, where as some bean species will lasts for hundreds of years if stored properly.
The only way to know will be to make an attempt at germination! You may be surprised at what will come up after that long.
Correction...that should have been "more than 10 years old", not 19 years.
I only have older nuts. Is it still possible to stratify? They are 2-3 years old. They have been stored at room temp in my home
Sure! It wouldn't hurt anything to try. Germination rate may be affected after that long of a period, though.
I used miracle grow potting soil but some plants are dying off after they start
Miraclegro will usually kill seedlings, and many plants.
I recommend any soil labeled as a "seed starting mix" as it will not contain much nutrients. This is what burns the young plants.
Soilless mixes are best suited for this. There are may options, and you can make them yourself! This article will help:
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/soilless-seed-mix-info.htm
How many square feet will 500 seeds cover
It depends on the seed! An easy way to calculate will be to look at the spacing on the seed packet, or ask the company that you got the seed from.
Divide your total area by the spacing requirement, an you can find the maximum coverage.
I have covered my newly growing rose plant which hasn’t sprouted yet with Saran Wrap. How long should I keep it covered
Normally, seeds are covered until they sprout.
https://www.gardeningknowhow.com/ornamental/flowers/roses/growing-roses-from-seed.htm