The best deterrent marigolds are the ones that are stinky. The more ornamental ones have been developed without much scent.
What I have used for pest detterrents have been garlic chives, garlic and onions. Garlic chives have purple flowers and are nice so people often use them around walkways.
Onion â€“ Onions and all members of the cabbage family get along well together. They also like beets,
strawberries, tomatoes, lettuce, summer savory and a sparse planting of chamomile. They do not like peas and
beans. Ornamental relatives of the onion are helpful as protective companions for roses. Since onion maggots
travel from plant to plant when set in a row, scatter your onion plants throughout the garden.
There are believers and non believers when it comes to companion planting. Most of the companions were discovered by people growing them together and learning from their mistakes. They found out through trial and error what works and what doesn't. Be flexible in your planning just because it didn't work out, it may not be your black thumb. You just haven't found the right plant for the right place yet.
Basic companion planting rules are:
Plant what is best suited to your soil conditions
Group together plants that have the same soil and water needs
All plants needs enough space to grow but you can interplant root crops like beets and carrots among shallower rooted lettuce. When you plant a tomato, you can plant lettuce, or other short season green between them since they won't need the space until they are older.
Fennel is good for the garden, it attracts a myriad of beneficial insects especially once it starts blooming. It repels fleas as well. But, it is not a good companion to most plants. I have mine planted next to ginger since they attract a different kind of aphid from fennel. Fennel also does not bother horseradish or gynuura (actually nothing really bothers them). Fennel and dill are aphid traps attracting aphids to them, so you don't want plants that have the same kind of aphid next to them. 10 ft away is fine. If you don't have an aphid problem dill and fennel do not impact other plants until they bloom, then they stunt plants near them. That is why dill helps tomatoes before it blooms but stunts the tomatoes once it does bloom.
The more diverse you make your garden the better it will be to protect itself. Planting companions and interplanting helps to confuse pests. Long rows of the same thing actually can cause problems if pests appear and can be harder to control since you have laid out a nice buffet for them they will be reluctant to leave.
Interplanting vegetables and flowers makes good use of the space and if planned well can be quite ornamental.
Other good flowers/vegetables to plant borage in early summer, nasturtiums in the cool season, sunflowers in spring and summer is ornamental and attracts bees and other pollinators. Flowering vines like scarlet runner beans are both ornamental and edible.
I do grow allysum as one of the basic plants to attract beneficial insects. It blooms six weeks from seed and blooms continuously for me all year. It is a low growing spreading mound so I grow it under my roses along with some garlic chives to defend the roses. I also have gladiolus interplanted among the roses. It was the best place that they liked and they have been popping up to bloom since 1989. I did have cuphea which also blooms profusely and attracts bees. I have recently planted geums in one of the border beds since it requires less water. The jury is still out on that one, it tends to spread.
Marigolds are annuals except for Mexican mint marigold, t. lucida, but it only blooms once a year and the flowers are small. I usually grow crackerjacks in spring and summer.
I let the African basil flower. It is a long lived basil and the bees love it. African basil is edible but has a lot of camphor so does not taste very good. It is resistant to basil downy mildew.
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Happy gardening in Hawaii. Gardens are where people grow.