The easiest search is when you already have a number for an existing patent. Some resources you may use are:
Although keyword searching is not the best way to find all patents related to one's idea (that would be classification searching), one can still benefit from a keyword search, using the following resources:
Before you begin, you may want to check out the video from the USPTO titled "How to Conduct a Preliminary U.S. Patent Search: A Step by Step Strategy."
You can search plant patents using many of the suggested sources on this site, but the majority will only have black and white images of these plants. The color is an important part of a plant patent; luckily there are options for viewing these color images. As a PTRC, the Business Library and Collaboration Commons at Texas A&M University has color plant patents in their collection.
One option available online is the below database from the University of Maryland that has scanned color images of plant patents from 1994-2009. The project is still in process.
Another database for color images of plant patents is the following resource from the New York Public Library's Science, Industry, and Business Library (SIBL), with images patents from 2012 to present.