I think it's too simplistic to say that there is inevitably competition for nutrients and rootspace and sunshine although it depends what you mean by growing optimally (and @daninthedirt did admit that may be acceptable). Now sure, if you're trying to grow some prize-winning tomatoes and nothing but those tomatoes, maybe the competition is harmful. And obviously two very heavy and dominant plants with high nutrient needs may compete to the (significant) detriment of both - but some combinations may work very well. It's not simple at all, it's not just competition - some plants need more or less sunlight, and partial shade from 'competing' plants may hardly affect some or even be beneficial. Some soil cover from other plants grown alongside might be beneficial. Plants don't generally use all of the nutrients in the soil, certainly not all at once, and they may have different needs at different parts of their growing cycle. The nutrient requirements are often described in terms where you look at which nutrient is the limiting factor (often nitrogen) - and for many of the nutrients and especially micronutrients, once they have 'enough' of that particularly input, adding more doesn't make any difference (and competition from other plants is not a factor); some plants bring their own nitrogen by fixing it at the roots. Likewise with root systems - root systems of different plants coexist all the time, and may even be beneficial to each other - helping soil nutrient exchange, drainage, tilth of the soil, etc. (Yes, of course, some root systems will outright compete with each other - you don't want to try to grow tomatoes in the dense woody root system of a big tree). Other things - microorganisms and fungi - are also present in healthy soil, and they're not all 'competing.' Now I'm not trying to push anything - just saying that 'competing for resources' is not necessarily the right way to look at it. It depends. In fact the limiting factor concept may apply well to most of the inputs - sun, nutrients, water - although water is probably the easiest to grasp, ie. once the plant has enough water it won't be affected by other plants taking more. I'm not a chemist but I think the way to look at nutrients in soil is not as if you're feeding a dog with a bowl of food (and the cat eating the dog's food is direct competition), but as a chemical solution - ie. as a percentage of the whole (the concentration), the total physical amount is not so much the issue because plants absorb nutrients differently. Some plant may grow best when (say) potassium availability is above X% (and below some other percentage); the difference to this way of looking at it is that the plant doesn't necessarily 'consume' all that much of the total potassium in its area, it absorbs enough and grows best by chemical exchange when the potassium concentration is in its optimal range (the plant's processes to absorb chemicals work best in that range of concentration, regardless of how much it actually physically needs in weight or volume terms). As long as the other plants don't outright consume so much that they throw off the concentration, it's not really a limiting factor. Note, I'm using potassium here a bit randomly - nitrogen is I think a bit more complicated, both because some plants require a fair bit and that nitrogen is more volatile and not as persistent in the soil. This is obviously a simplification as there are concepts like availability, the nutrients aren't in a liquid solution (soil is quite complex and heterogenic) even if overall it acts like a solution, may be partially/loosely bound to other chemicals, tied up by microbes, water is the most common solvent so moisture affects all of the above, aeration of soil matters too, etc., etc.

welcome to the forums ... it often helps to add big city name ... z5 alone is almost half the US ... and sometimes location helps ... they speak of the wild species ... you see the tall wild kin of these lining the roadways of MI in fall .... though not for the small garden.. they can make quite a statement en masse .... ken

The small brick half-circles are basically flower pots in the yard. Suitable for annuals and the occasional perennial and that's about it. Whatever suits your fancy. If you want to do landscaping instead, you need to make a 6' depth bed across the entire front. We don't know what the bed would do at the ends because we can't see them.

My first thought was tall Garden phlox as well. I have to go out and look at my obedient plant. I thought the leaves had more of a serrated edge and squarish stem.