As we know, NPK is nitrogen, phosphorus and potassium. Depending on what you are growing, is how you want to use the numbers. Basically, nitrogen is the first major element responsible for vegetative growth of plants above ground. With a good supply, plants grow sturdy and mature rapidly, with good, dark green foliage. Phosphorus is the second major element, essential for healthy growth, strong roots, fruit and flower developement and greater resistance to diseases. Potassium is the third element needed for the development of strong plants. It helps plants resist diseases, protects from the cold and during dry weather by preventing excessive water loss. If the soil is in good shape, compost and the like, you may not need much. For the lawn, you want more nitrogen, as you want more 'foliage' that's dark green. If too much nitrogen goes to the flowers, you will get more foliage but less blooms. Therefore, the need for the other two elements.

Hmmmm... good question. That's one of the reasons I often put multiple plants into a larger container, more room for error ;-) Don't take this as experienced advice as it isn't. This is just what I would try based on having grown the 3 footers in larger containers. I would say 10" pot would be on the small end of what I would use. Is this plant going to be narrow and tall or more bushy? Top heaviness is something you may want to consider when deciding container size. You don't want something that easily blows over all the time.

Phosphorus and to some extent Potassium build up in your soil when you fertilize and will stay in the soil for years. Nitrogen washes out every year. Fertilizer has three numbers: the first(N)represents the percentage of Nitrogen in the product. The second number represents the percent of (P) Phosphorus and the third the percentage of (K) Potassium. A balanced fertilizer would have the the same number for each. Examples: 5-5-5 or 10-10-10 or 20-20-20. The article you read recommended a product like 5-10-10 or 10-20-20. I said if you have fertilized heavily in the past use something like a 20-5-5 instead. You are wasting the extra money that the 5-10-10 costs if 20-5-5 works. That is the cheap lawn fertilizer. If you have not fertilized that area very much and have not built up the levels of Phosphorus and Potassium, use the 5-10-10. Or, if you use any of the fertilizers mentioned above, your dahlias will grow fine. How about just give them a little bit of fertilizer in June and in July and whatever you use will work. A plant takes up what it needs and leaves the rest in the ground. A farmer who fertilizes many acres has to be concerned about over fertilizing as it wastes money and pollutes the environment. If you used the 5-10-10 or 1--20-20 for five years or so, you will have built up levels of Phosphorus that will cause a decline in plant growth. You will also have decreased the Ph of the soil and may have made it so acid that the plants are affected.

Yes, Swan Island has an open house in the summer & they cut everything off at the same height so people can take pictures of the fields & they will be uniform. It will set the blooms back - you'd have to figure out how much bloom you want to give up & how long it'll take the plants to recover. Do you "stop" your dahlias when they get to about a foot tall? This is done early in the season to keep plants shorter & bushier & doesn't get rid of as many blooms as mowing them off later in the season. This article on the Colorado Dahlia Society site tells you how to do this. Here is a link that might be useful: Topping dahlias