"Even that is external to the seedless realization"
The analogy of the tree is used a lot through the Yoga Sutras. Specifically, it is used to illustrate how Karma unfolds in the mind, then manifests into our lives. Karma stores itself in the Chakras as "seeds" and will ripen when the right circumstances occur. The Vedic Astrology birth chart shows the timing of the fruition of the seeds, Yoga is the tool we use to stop them for sprouting. We have a lot of seeds planted in us, some will spout small plants, others will develop into gigantic trees, blocking us as we move along the path. In this Sutra, the seedless realization is referring to Samyama. In this state of awareness, no new seeds are planted, leaving us to tend the garden we already have within ourselves.
How do seeds get planted in the first place? It all happens when our senses trigger reactions to event. Most karmic events in life will pass us by without notice. Simple things like the food we decide to eat, the colors we choose to wear and people we choose to associate with happen at a deep level of awareness. It makes a lot of sense. If we had to actively monitor and consciously participate in every function of our being, we would never have time to do anything else. Nature throws us a bone and takes care of these things for us. In some circumstances though, these simple choices can cause us a lot of pain when then are interfered with by deep routed patterns.
For example, I was in a horrible car accident many years ago. To make a long story short, I was feeling sick and the next day I had to perform a critical function at work, I spent the whole night tossing and turning, thinking about how much sleep I could get in order to be fully alert and functional the next day. I ended up taking a lot of cold medicine, thinking that it would help me sleep more. The next day, I left for work, feeling sick and still woozy from all the cold medicine I took, and ended up getting into a bad accident. From that day forward, I developed a fear of not sleeping. Everything revolving around that event and set the tone for many years, changing my behavior in ways that I am still working on today.
Samyama has helped me start to get over the events surrounding those days. That accident is the tree that has sprouted in my consciousness. Seeing the event for what it was put me on the path of being able to tend to my own inner garden.