A ditch seed is when you implant the possibility of you ditching while plans are being made with a subtle yet effective 'out'. So when it comes time for those plans to happen or for you to make the decision whether you will participate or not, you can 'harvest' the now mature ditch seed that has grown and ditch with no consequences and the ditched will be understanding.
Friend asks: Hey can you help me move this Saturday?
You plant the ditch seed: Yeah man I could probably do that but I might have to work that night (whether this is true or not doesn't matter)
Saturday rolls around and you can choose to help your friend move or use your ditch seed and tell him you have to work after all and you won't look like a ditching douche.