Generally spiders are good for a garden, as they normally catch unwanted garden pests and eat them. So expect to encounter some spiders (and, consequently, spider webs) in the balcony garden. Some spiders are dangerous, such as the black widow spider, so if you're squeamish, scared or curious about the spiders that may visit your balcony container garden, try to identify the spiders you see, and research if they are beneficial or harmful.
Although it is very difficult to identify spiders (there are more than 37,000 known species of spider), you can closely observe a spider in your container plant garden and make an educated guess as to what it may be. Take a close-up photo of the spider and its web, and start your research.
The best guess when it comes to the Californian spider in these photos is that it is a Zygiella x-notata spider.
The first photo is of the spider in its large web, which was constructed quickly in the container garden. The closeup photo was taken after the spider quickly dropped down from the center of the web and hung out underneath the leaves of a jacaranda tree. In this picture, you can see the spider's markings, and long white/yellow legs.
The spider's body shape and color are very similar to the Zygiella x-notata, but the markings on the back are different. This spider's web is also a bit messed up, so it can't be identified by its web. Different spider species construct different webs (if they even construct them at all), so identification clues can come from the web's characteristics. Zygiella x-notata spiders build circular webs that have two sections missing, similar to a pizza with two pieces gone. Younger spiders are known to build these normally missing pieces, and if the web isn't strong enough, the spider will fill the empty spaces in. The missing pieces are in an upper corner where the spider has also built a "hideout." After wrapping and killing its prey, it brings its dinner to the hideout.
Zygiella x-notata is an invasive species in California, but it is not harmful to humans or container plant gardens. The only detriment of this spider in the balcony garden is its large web that gets in the way of the garden maintenance chores.