Trying to understand plant spacing...?

If one plant's spacing is 12 inches and another's is 18 inches, should there be 30 inches between the two plants? Or could there just be 18 inches? Or something else? Also, does the spacing mean that if the spacing is 12 inches, there should be 12 inches between the center of the plant and the pavement?

Trying to understand plant spacing...?
Plant spacing is from the center of the trunk/trunks or stem/stems of one plant to the center of the next. Generally, the spacing is slightly less than the spread so that the plants will grow together slightly to create a massing effect. For example, a plant that spreads to 24" diameter may be placed with a spacing of 18" to create a mass of that variety of plant. If you want to maintain some space between them, keep them spaced at the desired space, plus the expected mature spread. So to maintain 2' between plants, space them 4' apart (24" diameter + 24" space).





So with your example of the 12" and 18" spacing for plants, it depends on how you want them to look. If you want them to grow slightly together, plant them at 18". If you want to maintain some space between them, put them 18" + desired space.
Reply:Spacing is determined by the size of the plant when it is full grown. Every plant needs that space for aeration and non- competition. The plant tag or the seed package should tell you all the info you need for each individual plant. Like how tall, how much space, how many days to maturity, whether it like full sun or shade and how much water it likes. Some plants like tomatoes might need to be staked to help them stay upright. There are tomato cages out there, but I like wooden stakes placed in a pyramid and tied at the top with some kind of strong twine. The stakes need to be pushed into the ground as far as they will go , so they don't fall over with the weight of the full grown plant. Adjust as the plants grow bigger. As far as the 12 and 18 in. spacing, just keep in mind the size of the plant when its full grown. If you don't know ask a garden shop employee or check out these web sites: garden.org, gardenweb.com, learn2grow.com. Good luck! If you need more help email me.