What would it look like? How would it be powered, and how would it transmit power to the parts that need it? What actions would it be capable of performing?
There's no single, right answer to these questions. Rather there's a wide range of potential answers, some of which will likely prove more workable than others. Let's look at some of the possibilities.
What would it look like? Almost anything, from a snake-like device slithering along the surface, to what appears to be little more than a single wheel rolling about, to a platform supported by long, spider-like legs, to a beam supported at each end by wheeled trucks. It may turn out that the best arrangement is a mixture of larger and smaller machines, with the larger ones designed to never put their weight on soil being cultivated.
How would it be powered, and how would it transmit power to the parts that need it? They could get their power directly from the grid, from engine-driven generators, from wind generators, from photovoltaic panels, from concentrating solar collectors, or simply from batteries or other energy storage. Any such machine will need at least a small amount of electricity, to power the electronics. Mechanical power could also be electrical, but needn't be. It might be provided via compressed air. Delicate, articulated parts might be moved via fine cables or wires, much as our own fingers are moved by tendons linking to muscles in our forearms.
What actions would it be capable of performing? Planting seeds, of course, beyond that the possibilities are nearly endless, but even the placement of seeds can be accomplished in many ways.
In conventional agriculture, seeds are typically inserted into the soil in rows, through an opening created by a disk (a rotary knife), and covered over by a roller. This is an efficient method of planting a large area to the same crop, or even to a mixture of crops with seeds of approximately the same size, if you don't mind running the planting device and the tractor pulling it over the same soil surface through which the seeds will have to sprout and in which the sprouts will have to grow. Most such planting devices require the soil first be prepared into a seedbed, meaning that plant debris from previous crops must either be turned under, with a plow, or broken down by a combination of tillage, decay, and weathering, to form a relatively uniform surface, easily broken into small particles. A few such planting devices are capable of placing seeds through rough plant debris, rendering the preparation of a seedbed unnecessary, but they're still mainly used to sow a single crop to a large area.
A robotic gardener would also need to be able to place seeds not only through plant debris but between standing plants. It would do so one at a time, perhaps very rapidly, like a sewing machine, but still one at a time, and far more precisely than any bulk planter, positioning them in the most advantageous microenvironments available. Even when planting in bulk, the use of rows would be optional, and in many cases a honeycomb-like pattern might prove preferable.
Weeding might be accomplished by identifying weed seedlings and removing them while still in the sprout stage. Undesirable plants that propagate by root spreading could be controlled by injecting steam below the surface, wherever they appeared. Insects, like aphids, could be controlled by removing infested leaves. Diseases could be controlled by removing infected plants. Nutrient deficiencies could be identified early and treated quickly. If plant debris needed to be reduced to mulch it could be clipped off at ground level and shredded, without disturbing the soil.
In each case the action taken would be local and specific, rather than applied to an entire field, and generally would not involve moving large amounts of soil around, a practice which wastes both energy and soil fertility.
But the essential requirement, without which this whole scenario would be futile and meaningless, is that the machines must operate autonomously, puttering through their days without constant human supervision. They must have both the ability and the latitude to choose what to do next for themselves. Considering they must also operate in uncontrolled environments, this is the greatest challenge.