Leaves, sticks, and any other small plant debris will have no immediate harmful effect on your system. They will usually fall to the bottom of the condenser. The condenser of a AC system is designed for outdoor use, and so there's no components of the system exposed that would be effected by such outdoor conditions.
Though it is recommended that the condenser be routinely cleaned out. Depending on your area and the environment, plant debris could possibly shorten the life of your system. Leaves bunched up against the coils could degrade them over time, or hold water as they decay and cause rusting to the unit. These problems happen over an extended period of time, and are avoided with routine maintenance and cleaning.
Try contacting a recommended local AC company and asking about any service agreements they offer. Many companies will offer annual preventative maintenance and cleaning; call around and research the local contractors in your area to find one you trust.
You can clean the condenser yourself, although disassembling any of the system is not recommended if you are not experienced with servicing AC units. There is danger of electrical shock or injury from the components; the equipment can also be damaged from improperly handling or disassembling. It is recommended that an experienced service tech do any interior cleaning of your equipment. See the FAQ about Cleaning Coils for more info.
Many products are offered that will prevent most debris or leaves from falling into your system. There are several different types; such as a cap that lifts when the system is in use but covers the top of the system when it isn't running, or screens that prevent leaves or sticks from falling into the condenser. Do some research and ask a local AC contractor their opinion, many of the products are offered for purchase online and are easily installed.Tags: debris, fall in condenser, Leaves, protect, screen