On its own, the word plantingly is not a "real" word (not much evidence of use, not in dictionary). However, in this particular sentence, plantingly is being used in the phrase "plantingly challenged". This is a phrase that was formed by analogy with other phrases of the form "__ly challenged", for example:
- mentally challenged (mental difficulties)
- academically challenged (academic difficulties)
- financially challenged (financial difficulties)
- socially challenged (social difficulties)
In the case of "plantingly challenged", the phrase is supposed to mean that the person has difficulty with planting. The -ly suffix was added to this word in order to maintain the form of other "___ly challenged" phrases (and in this case, the use is intended to be playful or humorous).