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Novice planning a border - help please!
Hi all 
I'm planning a new flowerbed across two thirds of the front of my lawn, approx 4mx1.8m (but curved) and I want it to look restful with airy plants in subtle-ish colours, blooming throughout late spring/summer. I'm planning a small water feature (solar fountain in some kind of bowl) and I've settled on a few main plants I like:
Perovskia
Yellow achillea
Stipa gigantea
Heuthera obsidian or similar
Mexican fleabane (erigon)
My worry is the sizes of the first three, especially stipa gigantea. My lawn is about 10m deep and 8m wide with shrubs round the edges (some quite large), we also have a 3m deep patio between the lawn and the house so will usually be viewing this flowerbed from the patio or the sitting room (i.e. sitting down), but it also needs to look nice from the back when you walk around it into the rest of the garden. Because of that I want to mix up the heights a bit, hence I wants relatively see-through plants so as not to block the view of the rest of the garden.
Perovskia "Blue Spire" is lovely but I think too tall at 1.5m, "Little Spire" is possibly a bit small at 60cm? Also not too sure if perovskia is a bit dense (only a problem with the taller one probably). I've found a compact achillea variety called Moonshine which gets to 60cm which is fine. But stipa gigantea is, obviously, gigantic and though it's lovely and airy I'm worried it might look way too big at the front of the lawn. There are a couple of smaller varieties but I think they're very new as I can't find them anywhere online to buy.
What do you all think??? I don't want to make loads of work for myself in keeping things small that really want to be tall, so any reassurance about sizes, or suggestions of alternative varieties or even different plants that have a similar look would be much appreciated!
Thank you!

I'm planning a new flowerbed across two thirds of the front of my lawn, approx 4mx1.8m (but curved) and I want it to look restful with airy plants in subtle-ish colours, blooming throughout late spring/summer. I'm planning a small water feature (solar fountain in some kind of bowl) and I've settled on a few main plants I like:
Perovskia
Yellow achillea
Stipa gigantea
Heuthera obsidian or similar
Mexican fleabane (erigon)
My worry is the sizes of the first three, especially stipa gigantea. My lawn is about 10m deep and 8m wide with shrubs round the edges (some quite large), we also have a 3m deep patio between the lawn and the house so will usually be viewing this flowerbed from the patio or the sitting room (i.e. sitting down), but it also needs to look nice from the back when you walk around it into the rest of the garden. Because of that I want to mix up the heights a bit, hence I wants relatively see-through plants so as not to block the view of the rest of the garden.
Perovskia "Blue Spire" is lovely but I think too tall at 1.5m, "Little Spire" is possibly a bit small at 60cm? Also not too sure if perovskia is a bit dense (only a problem with the taller one probably). I've found a compact achillea variety called Moonshine which gets to 60cm which is fine. But stipa gigantea is, obviously, gigantic and though it's lovely and airy I'm worried it might look way too big at the front of the lawn. There are a couple of smaller varieties but I think they're very new as I can't find them anywhere online to buy.
What do you all think??? I don't want to make loads of work for myself in keeping things small that really want to be tall, so any reassurance about sizes, or suggestions of alternative varieties or even different plants that have a similar look would be much appreciated!
Thank you!
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Posts
From my experience, to have a border that could be admired from all angles, you need to try and cater for that by planting instead of clumps, but in strips/waves. Obviously, each plant has their different heights and habits, but if you loosely think along those lines, when moving around the border, you see more definite form and stronger and bolder statement. That also means, you can plant taller things further out and smaller things in-between. Also think about leaf shape against other leaf shapes.
A plant that reaches 1.5 meters may feel huge but over time, the more you garden, you will be amazed by how many perennials can easily reach those heights. Example 50% of Perovskia will be the airy see-through spires. Same with plants like Achilleas.
Forgot to mention that my soil is acid clay - I know clay isn't not ideal for some of the plants I mentioned but am planning to did down and loosen the soil, and mix in compost and lots of gravel, so hopefully that will help...
Put in the plants, if they don't work, you could always dig them out and move somewhere else in spring/autumn time. With heights, they are just loose guides. You will find that from year to year, your plants may vary in growth because the weather and temperature can affect their size.