I have 3 beautiful calla lilies which are in containers. Could you please explain why the flowers are starting to close up?
This article gives you some possible answers to consider: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/make-calla-lily-bloom.htm
How long in the summer should I expect my calla lilies to continue to bloom?
A calla lily variety may flower for from 4 - 10 weeks, depending on the specifics of each one. Also, they bloom at different times, so you can plant several varieties to have flowers from late spring all the way through summer. This article will tell you more about growing them: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm
By the way, to learn your zone, you can go here: https://www.gardeningknowhow.com/planting-zones/
When my calla start to die back, can you cut the dead lilies back?
You can cut the flowers, but allow the foliage to fade and die back naturally. This article has more information: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm
What do I do with the growth in the base of the flower?
Not sure what you mean. The growth at the base of the flower stem, from which all the stems and leaves arise, is the rhizome, or underground stem. This is the part you divide to make more plants, or dig up and overwinter inside if you live in a cold area, then plant again in the spring: This article has information https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm
It occurs to me that you might mean the rod-like shape around which the white petal curls. That is called the spadix, and it contains the actual flowers, masses of extremely tiny little things. The white "petal" is not a petal but a modified leaf, called a bract or spathe
The leaves and flowers are turning yellow and drooping. They feel almost like they're rotting.
This article contains information that should help you find the cause of your problem: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/drooping-calla-lilies.htm
I planted some calla lilies in partial sun and none have bloomed. Lots of plant just no flowers. I planted one in full sun and it had 2 flowers on very short stems-they were hard to see for the foliage. I fertilized them once with 10-10-10 fertilizer. They all look very health just no flowers.
Figuring out why a plant is doing, or not doing, something is a process of elimination. Here are a number of possible reasons for you to investigate: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/make-calla-lily-bloom.htm
I have two lovely calla lilies bordering my small garden pond. Last year I had one flower on each plant. This year they have gone mad, 9 on one and 5 on other. My question is what do I do when the flowers wilt and die? Leave to nature or cut the stems to ground level? They're a bit ugly in their wilting mode. Many thanks c.e dyke
Congratulations on your happy callas! Certainly, go ahead and cut them stems when the flowers are spent, unless you want to grow seeds. Here's an article which may contain some information you can use: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/calla-lily-seed-info.htm