I have some herbs growing in some outdoor planting containers, however there are no drainage holes in the containers. There is a plastic meshing at the bottom that creates a gap between the bottom of the container and the soil, however there is water gathering in this gap. Will the water eventually evaporate or will it cause issues? I'm not sure if I can create any holes in the containers. The plastic is very sturdy.
Yes, you should drill some drainage holes or you risk root rot. In order to do it safely with an electric drill, you should empty the container and turn it over to drill the bottom. Use a 3/8" to 1/2" drill bit for 4 to 6 holes. If that's not available you can try using a screwdriver to jab some holes in it.
https://www.gardeningknowhow.com/special/containers/container-gardening-supplies.htm
Just starting out some indoor gardening in a small apartment, and only have window sill space pretty much. I was reading about pots/containers to use and I found out about grow bags. They seem to be better in almost every way, so I was wondering if they are OK to use on a window sill or if there's anything wrong with using them instead of a pot, or like pros and cons of both. I could also use some general advice too if you have any about container gardening indoors too! Thank you very much.
Grow bags may be okay on a window sill as long as they have drainage and you place a saucer to catch excess water.
https://www.gardeningknowhow.com/special/containers/grow-bags-for-gardening.htm
Here are articles on indoor container gardening, and there is a lot more, including YouTube videos, if you search the Internet with that term: "indoor container gardening."
https://www.gardeningknowhow.com/houseplants/hpgen/determining-the-best-plants-for-your-indoor-containers.htm
A plant pot is described as 3 and a half inches. Is that across the top or the depth?
How many seeds to be planted in a 1foot dia cement vessel? Should this seed be planted in ground only ?
It really depends on the plant in question. I cannot give much information without know the particular plant as each can have different spacing requirements.
new baskets just made up
It depends on what is in your baskets: perennials, tropicals, annuals.
I looked up the average weather in Leeds, England, and the warmest months are June, July and August. April and May are cool months but above freezing. I would wait till June to put out tropicals but anything else you could put out now, as long as it doesn't freeze.
https://www.gardeningknowhow.com/special/containers/winterizing-hanging-baskets.htm
https://www.gardeningknowhow.com/special/containers/watering-hanging-baskets.htm
Can I use it for planting vegetables outdoors? Is it safe? Can I make it safe?
It should be fine. Make sure your container has drainage holes. If not, drill plenty in. You can always line the cover with plastic before adding your soil. I did that with my wood planter. You need to punch holes in the plastic, too.
Zone 8 have tons of ones already would like to use what I have to make some spectacular container gardens.
Sure, these articles will help:
https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-8/zone-8-plants.htm