If you want visitors and passers-by to be amazed at your green thumb, plant zinnias.
Image from Park Seed. I'm sure they won't mind me using their photo if you'll consider buying some zinnia seeds from them
There's nothing easier to grow in the world. Seedlings emerge quickly when planted early indoors for later transplant, or when direct sowed into warm soil in the spring. The seeds like it moist and warm while they're germinating, but once the plant is a few inches tall it's a hardy little critter that will tolerate hot and dry conditions. In fact, they'll do better when it's not especially rainy as they are prone to powdery mildew.
I totally ignored this old fashioned flower until 7 or 8 years ago because I thought they were somewhat... boring. But I ran across a pack of a specialty variety that intrigued me and succumbed. I threw out some seeds in my raised bed veggies thinking they may attract pollenators, and luckily my instinct was right. The seeds emerged quickly, the plants grew and flowered incredibly fast. I made a bouqet for my grandmother-in-law who was in the hospital at the time and she was just delighted to see this old fashioned flower by her bedside. A second visit over a week later and the little bouquet looke as good as the moment I had picked them days earlier.
I noticed that after cutting them they branched out; the plants got bushier and produced even more blooms. I started filling mason jars with them and putting them all over the house. The flowers came all summer long, right up until frost. And of course, I saved all the dead flowers at the end of the season in hopes that I'd have free zinnias the next year. I got 2 gallon bags full of blooms dried and saved, so I was thrilled that I could save at least $3 on seeds the following year.
What I didn't know at the time is that zinnias easily cross-pollinate, so the following year I got TONS of new colors and types from what I had the year before. That first year the flowers were the double variety and there were probably only 6-8 colors. By year two I had dozens and dozens of colors and color combinations, from muted pastels to vibrant oranges and reds. Single rows of petals, double and even triple bloom varieties.
I've grown my own zinnias every year since from the seeds I save, and am surprised and delighted with what comes up each year. I love them so much that I splurge and get a new variety or 2 of seeds to try out. One year I got a packet of "green envy" which is a pea green bloom. One year I got some with ruffled cactus blooms. Last year I grew some striped ones.
Since zinnias are my mom's favorite annual and I have a greenhouse this year, I got 5 or 6 new varieties that I'm growing to surprise her with. Interestingly, the store bought seeds have not been nearly as quick to emerge and grow as the ones I'm growing from my own seeds. By growing and saving your own seeds over a period of years, the plants become better acclimated to your own garden and you'll get better and better flowers.
It's easy to save these seeds but tricky to explain how to do it, so I'm sharing a video below to give you direction. Thanks go to Jane Diedrich for this helpful advice!