After watering a potted plant, the tray the pot is on sometimes has a significant amount of leftover water in it. It's usually got some soil in it and occasionally gets a light tea color to it. Can that leftover water in the tray be poured into another pot to water a different plant?
Are there any exceptions/guidelines to this (e.g. not with allelopathic plants, or only for two pours/rounds before the leftover water should be poured onto the ground)?
Agreed that it can spread disease among your indoor potted plant.
You should do like me and have a set of water buckets somewhere in order to collect this with green water. Use this extra left-over water to pour on outdoor plants -- if you have any -- and rather when they need it. I prefer to use limited size water buckets to avoid water to stay too long and to easily clean the thing.
And I'm not afraid to have this kind of "microbes and fungus" culture poured on outdoor plants: for me, soil is a place for life, and adding mixture on top of it add more life to it.
Technically yes, but it's a good way to spread pests and fungus etc from one plant to the other that are in the soil. The thing is what can happen if the soil gets dry is it runs straight down the sides and out the bottom, only later does the water wick its way back up.
If you've used sterilized potting soil, which all plants in pots deserve my opinion of course, you bet you can. If you are using garden soil in your pots it would be best to not reuse. Never allow your plants and their pot to sit in the puddle of water in the saucer. I've seen and understand nurseries that during incredibly hot weather sit their pots in water. But that is the only instance. In fact you should raise the bottom of the pot about a 1/4' from the surface of the saucer or any surface the pot drains upon. Use tiles, flat rocks even specially made 'pot feet' some that are made to look like tiger paws, or frog feet. That really improves drainage.
The soil that comes out tells me you've newly transplanted your plants into new pots and hopefully sterilized potting soil. That stops very soon. I've told people to use old nylons or chunks of old nylons you know for women's legs? to cover the drainage hole. That works well to keep soil in its place. Don't use rocks or gravel or pieces of clay pots or packaging peanuts above the drainage hole as that creates a perched water table and causes very bad drainage. Another discussion.
If you are not using sterilized potting soil just dump that stuff in your sink or out of doors. Plants in pots are totally dependent on their human stewards. Give them the best soils, as much light as possible (careful with this as plants used to low light have thin epidermis that won't protect them from sunburn), fertilizer in the correct amounts/formulations and drainage. Plants are tough and sure they will live but they won't be healthy and vigorous.
Do you have water restrictions? I also never use tap water for my indoor or outdoor plants. Of course I have well water. But indoor plants should have distilled water you purchase. Otherwise, the salts in the water will cause burned tips and margins. Filtered tap water would be great.
I wouldn't bother saving the 'saucer' water to use for my other plants. What are you doing for fertilizer? If you use water from fertilized potted plants that water will have fertilizer chemicals and just a wee bit too much fertilizer will kill plants sure and certain.
