Some plants just aren't compatible.
Some plants don't like growing in close proximity to other plants, for a variety of reasons. Here are examples of what plants don't qualify as 'companions':
Moisture-loving and drought-tolerant plants. Too much moisture can create root-rot in drought-tolerant plants.
Gardenias with anything - they're known for chronic diseases and they spread the diseases to plants around them, killing them all.
Garlic with other vegetables - garlic builds up sulfur in the soil, and too much of a good thing can stunt the growth of plants that don't need extra sulfur.
Knockout roses and Annabelle hydrangeas - while they look fabulous next to one another in flower bouquets, in the ground these two have different growing requirements. Roses love a lot of sun while hydrangeas prefer shade. Both like to fill a space and have different types of root systems, so you don't want them to compete with each other or neither will thrive.
Tomatoes and potatoes - they are in the same family and will compete for nutrients and get (and share) the same diseases i.e. blight. You'll also want to keep tomatoes growing away from cabbage and broccoli and any other crops in the Brassica family.
Mint and everything else - mint is best suited for container planting as it quickly takes over a space and outcompetes anything around it.
Monarda (Bee Balm - shown in photo above) and Cucumbers and/or Squash - Bee balm is unfortunately susceptible to powdery mildew (a persistent fungal disease), and you do NOT want this spreading to your cucumbers or anywhere in your garden. Plant Monarda in a container or well away from cukes. The plant itself is a good pollinator which is beneficial to cucumbers, just not in close proximity.
Moisture-loving and drought-tolerant plants. Too much moisture can create root-rot in drought-tolerant plants.
Gardenias with anything - they're known for chronic diseases and they spread the diseases to plants around them, killing them all.
Garlic with other vegetables - garlic builds up sulfur in the soil, and too much of a good thing can stunt the growth of plants that don't need extra sulfur.
Knockout roses and Annabelle hydrangeas - while they look fabulous next to one another in flower bouquets, in the ground these two have different growing requirements. Roses love a lot of sun while hydrangeas prefer shade. Both like to fill a space and have different types of root systems, so you don't want them to compete with each other or neither will thrive.
Tomatoes and potatoes - they are in the same family and will compete for nutrients and get (and share) the same diseases i.e. blight. You'll also want to keep tomatoes growing away from cabbage and broccoli and any other crops in the Brassica family.
Mint and everything else - mint is best suited for container planting as it quickly takes over a space and outcompetes anything around it.
Monarda (Bee Balm - shown in photo above) and Cucumbers and/or Squash - Bee balm is unfortunately susceptible to powdery mildew (a persistent fungal disease), and you do NOT want this spreading to your cucumbers or anywhere in your garden. Plant Monarda in a container or well away from cukes. The plant itself is a good pollinator which is beneficial to cucumbers, just not in close proximity.