Ok then, thinking off the top of my head...
its the size that you need to use in your discount calculation.
you need to add a column for size to your ecart object, so when you have items in the cart array, you can use this to determine how many of each size you have.
presuming all plants are the same price, and that you can mix and match any combination of plants...
take 15-29 plants
the pseudo logic would be
if the number of size 'plug' is greater than or equal to 15 and less than 30, then apply a flat rate discount of 0.15 * the number of size 'plug'
so for 20 plugs, the overall discount would be 0.15 * 20 (£3.00), which equates to £1.00 per unit
This will give 18 discount rules which, once set up, will work fine, but take more effort to maintain if the values change regularly, or if the customer needs to have conrol over the discounts.
In this instance, you could store the discounts in a database table and interrogate this instead, but i'll leave that for now.
Hope this helps