If you have plots that are spatially correlated and observations on plants within a plot that are temporally correlated, then you can try something like
random plot / type=ar(1);
repeated time / subject=plant(plot) type=ar(1);
Unless you have a lot of plots, the estimates of the components of the AR(1) structure of the spatial component of your design are going to be rough at best. If you have measurements of the plot locations in two dimensions, then you can try TYPE=SP(POW)(x y) on that RANDOM statement to incorporate the two dimensional distance between the plots.
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