I've started testing plot layouts. I've found that you can place plots right next to each other, but when you go to plant, you can't put more than 2 seeds in adjacent plots. You can create a line of plots/seeds, but the rows have to be seperated or else it won't let you place the seeds. I'm not sure if this is a bug or not, but I believe that either, seeds should be able to be planted in adjacent plots, or that plots shouldn't be able to be placed if a seed is not able to be planted there.
So far I've only tested outdoors, but when I get some more energy, I'll move my experiments indoors.
You need to leave enough space between rows to walk in between. That will make planting easier and when its time to harvest you must be standing close to it. If you put them all together with no room to walk you will not be able to get close enough to harvest them.
You should also consider that the higher level spells cover areas of plants rather than a single plant. Planning is an issue to consider at level one gardening, or you will be using the 'remove plot' spell a lot.