Designing/Planting a Hosta Garden According to Your Needs
I have read a lot of posts here in Hosta Forum asking how much space to leave for planting a certain Hosta. I have had the same question myself, and my answer has always changed over my years. My answer now is, "whatever you want from your garden."
-Appearance: I have always liked natural looking gardens and in nature few things grow with large open spaces between them except in deserts, etc. So I prefer shoulder to shoulder spacing. Mulch is low priority.
-Spacing: I will plant to leave just enough room for optimum growth in the 3rd year to fill the gap. Often they won't need moving for many years. Other times they may remain in the spot for 3-4 more years before looking severely overcrowded. Sometimes my hosta doesn't meet expectations. I can always maintain the desired size of a fast growing hosta by taking divisions for potting as gifts, etc.
If you HAVE to move one, see ken's tape and move system.
-Weeding: The older I get the less time I have wanted to do weeding, and then I want to do it the simplest way possible. (Again, kudos to ken for his discarded mustard jar for use in dripping Round-up on tough to pull weeds, saplings, etc.) Mature hostas make better weed barriers than the best mulch, in my opinion.
-I wouldn't leave a 5 foot space for any hosta that I didn't DESIRE to fill in 3 years (the green banana factor applies). I have a spot in the front of my house that is perfect in my eyes for a huge Sagae. I have reserved a 6'x6' (36 sq. ft.) space for it. Around it are Poppies and Surprise Lilys whose foliage fades about the same rate as Sagae grows out each spring. In mid-summer when the Lilys bloom I hope they will poke through the hosta leavesin mid. Maybe I can post a Sagae which appears to have huge pink flowers. (lol)
I say all this because my age doesn't leave me with a lot of energy for higher maintenance gardening. So one of my beds is bordered with a wall of really low maintenance sedum which I can mow along on my rider. My gardens won't compete landscaping-wise with the best, but they suit me.
Everyone needs to find their own level. I have seen people give up on hobbies because they haven't found their personal level ground. No-one should have to do this. If you want a giant, fast growing hosta and only have 9 sq. ft. of open space available, get and plant it. If you have to change it out in 4-5 years, no big deal. You may have it worked out by then, like moving out neighboring plants to make room for it - if that is what you want.
Gardening is a self-perpetuating, gratifying hobby. Live it to your fullest.