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The Best Annual Flowers for Full Sun

Heat and Humidity are No Problem

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It gets pretty hot sitting in full sun all day. Some plants love it, some faint. Humidity can add to the oppressiveness of hot days, but even dry heat can be uncomfortable, when the temperatures top 90 F. People have air conditioning to escape to, but our plants have to learn to adapt. Perennials with deep tap roots and water conserving leaves handle the heat of full sun well enough. But annuals never have a chance to develop an extensive root system and they already expend a lot of energy growing rapidly and flowering profusely.

You have to choose wisely, to find annual flowers that can stand up to extreme hot weather. With dry heat, a little afternoon shade and a good layer of mulch will help sustain many plants, but you don't want to be faced with constant watering. And in areas with high humidity, there are a host of additional problems waiting to pounce, especially if the heat and humidity persist through the night. These stressed plants are sitting ducks for insect pests and fungal diseases.

The following 12 plants can stand up to heat and humidity. You will want to get them established, before the worst of the heat sets in. And you will still need to keep them watered regularly. But they won't faint during the day or require a lot of additional care.

I've listed them as annuals, but as with most plants considered annuals, they are actually tender perennials. They cannot survive freezing temperatures and are grown as annuals in many climates. I've listed the USDA hardiness zones, in case you live in a warmer area.

1. Amaranth (Amaranth spp.)

Amaranth in Bloom
Photo: © Marie Iannotti
There are many varieties of amaranth. Some are grown strictly as food, some for their leaves and others for use as a grain. Amaranth is the most widely grown grain in the world. However gardeners love them for their chenille like blooms or colorful foliage. Love-lies-bleeding (Amaranthus caudatus), with its long, dangling, mauve-pink flowers, and Joseph's Coat (Amaranthus tricolor), with splashy red and yellow leaves, are two you may be familiar with. Amaranth plants can grow from a few inches to 8 ft. tall. They all grow well from seed and can handle just about any growing conditions.

USDA Hardiness Zone 10 - 11

2. Celosia (Celosia spp.)

Celosia Flowers
Photo: © Marie Iannotti
If you notice a similarity between the flowers of Celosia and amaranth, it's because they are both in the same family. The name Celosia comes from the Greek word for burned, because the flower heads of Celosia argentea (Syn. C. plumosa) look like flames. The colors are certainly brilliant. However there are other species of Celosia that are popular in gardens. Celosia crestada, also called crested celosia or cockscomb, has rippled flower heads that look like a rooster's cockcomb. Celosia spicata has much more subtle spiky flowers that are likened to spikes of wheat. And there are new hybrids being introduced all the time. The flowers remain attractive for weeks and most varieties also make great cut and dried flowers.

USDA Hardiness Zones 9 - 12

3. Cleome (Cleome hassleriana)

Cleome Flower
Photo: © Marie Iannotti
Cleome's common name is Spider Flower, for the long "legs" that jut out from the blooms. The plants start flowering from the bottom up, lengthening the bloom period. As the flowers fade, long, slender seed pods form. Cleome are prodigious self-seeders, but because most are hybrids, you never know what colors you will get next year.

Seed can be direct down, anytime after your last frost date. These are tall flowers that branch out and generally can support themselves without staking.

USDA Hardiness Zones 8 - 11

4. Cosmos (Cosmos bipinnatus)

Photo: © Marie Iannotti
Cosmos flowers are about as easy to grow as it gets. You can find them in rich, bright shades of pink, purple, orange and red, soft pastels and even white. The flowers are only about 1 in. across, but they just keep coming. The plants are sturdy and intermingle well with other flowers. This is another eager self-seeder, but not to the point of nuisance.

Different varieties will grow from 1 - 4 ft. tall. Cosmos are easy to start from seed. Direct sow them anytime after your last frost.

USDA Hardiness Zones 9 - 11

5. Cupflower (Nierembergia linariifolia)

Nierembergia
Photo Provided by Proven Winners (www.provenwinners.com)
These delicate, charming flowers are in the nightshade family. The difficult to spell name, Nierumbergia, is for the Spanish Jesuit and mystic Juan Eusebio Nieremberg. Oddly Nierumbergia remains more popular than its common name, Cupflower.

Nieremburgia is extrememly popular in containers, but it's perfectly at home in the garden and makes a nice edging plants. It has a clumping growth habit and quickly fill out. As with so many plants

USDA Hardiness Zones 7 - 10.

6. Cypress vine (Ipomoea quamoclit)

Cyprus Vine (Ipomoea quamoclit)
Photo: © Marie Iannotti
Tubular, star-shaped, red flowers and thin, ferny leaves makes Cypress vine a very ornamental climber. It's in the same family as Morning Glory and grows just about as quickly, reaching 10 - 15 ft. in no time. It doesn't take as long as morning glories, to start blooming, but it will grow vine before it starts setting buds, so prepare to be patient. I like to start them indoors, in peat or paper pots, a few weeks before my last frost date. But you can direct seed outdoors after danger of frost.

Be sure to give them something to climb or sprawl over. They will grab hold of other plants, but if you are using a trellis, you will need to help them get started, twisting around it.

USDA Hardiness Zones 7 - 11

7. Lantana (Lantana camara)

Photo: © Marie Iannotti
If you're lucky enough to live in a climate where Lantana is hardy, you probably have it growing as a shrub. It also trains easily into a standard or small tree and I've seem many gardeners in colder climates do this and over-winter the plants indoors. Lantana flowers are often bi- or tri-colored in wonderful sherbet shades. As with all the plants listed here, they bloom throughout the summer. All parts of this plant are poisonous and can cause skin irritation, so handle with care.

USDA Hardiness Zones 8 - 11

8. Marigold (Tagetes spp.)

Photo: © Marie Iannotti
They're so ubiquitous we don't give marigolds their due. These are extremely tough little work horses. They do best in full sun and when grown a little on the dry side. If crowded in damp conditions, they can be prone to mildew, but you can avoid that if you give them plenty of air flow. Deadheading will get them blooming repeatedly, but even if you don't bother with it, they will resume blooming soon enough.

Another under-rated feature of marigold is their use as companion plants, repelling pests like asparagus beetles, bean beetles, nematodes and even rabbits.

USDA Hardiness Zones 9 - 11

9. Ornamental Hot Peppers

Growing Ornamental Hot Peppers
Photo: © Marie Iannotti

OK, these are not technically grown for their flowers, but they're just as beautiful. Ornamental peppers are edible and most are extremely hot. They are also generally small and difficult to harvest, but someone noticed how beautiful they are and thought to put them in the flower garden. I have had problems with small pepper seedlings being eating by rabbits, when I put them in an unfenced garden, but once the stems begin to harden off, they are pretty much safe.

Just like the peppers we grow in the vegetable garden, ornamental types go through several different colors, as they ripen. Some, like this tri-colored variety, has multiple colors on the plant at any given time.

USDA Hardiness Zones 9 - 12

10. Tithonia (Tithonia Rotundifolia)

Mexican Sunflower (Tithonia)
Photo: © Marie Iannotti
This might not be what you'd expect to see, if you planted sunflowers and I haven't noticed the flowers following the sun, but Tithonia, or Mexican sunflower, is definitely a sun lover. Give it a hot, sunny sight and it could easily reach heights of 5 - 8 ft. It is definitely a back of the boarder plant and may need some support or staking, especially in windy sites.

You can direct sow seeds, after danger of frost, or start them indoors, 4 - 6 weeks before you set them out. Don't rush them. The seedlings can stunt if exposed to cold temperatures.

USDA Zones 8 - 10

11. Verbena (Verbena spp.)

Rose Verbena (Glandularia canadensis)
Photo: © Marie Iannotti
There are several species of verbena that make great garden plants. Most start blooming early in the season and continue on until frost. There are low growing , ground covering verbenas, tall, airy Verbena bonariensis and upright Verbena rigida, which grows about 3 ft. tall.

Verbena can be prone to problems like botrytis, if it is grown in damp conditions. Although it needs moist soil to become established, once it has settled in, drier conditions will keep it happy.

USDA Hardiness Zones will vary with variety, but they tend to be short-lived as perennials.

12. Zinnia (Zinnia spp. and hybrids)

Photo: © Marie Iannotti
Finally, a true annual. Zinnias are native to Mexico and Central America. They truly love heat and bloom so easily, you can use them as a cut flower and they will simply bloom again, in a day or two.

Older varieties are prone to powdery mildew, in damp or humid weather. It doesn't stop them from flowering, but it does make the foliage look unattractive. Some of the newer series, like Profusion and Zahara, don't have that problem. Their flowers tend to be a bit smaller, but just as abundant. And there's a zinnia color fro everyone, from peppermint stripes, to eye-popping golds to delicate neutrals.

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