Tall Annual Phlox: The Complete Growing Guide for Tennessee Gardens
The tall annual phlox (Phlox drummondii) is a heat-loving flowering plant reaching 1 to 3 ft (12 to 36 in) in height․ It blooms from late spring until the first frost․ Native to Texas and well suited to our climate‚ they can withstand the humidity of summer better than most perennial species of Phlox․ Their five-petaled flowers come in almost every color of the rainbow‚ except blue․ If you're looking to add color to your garden bed and you can't wait several years‚ this is one of the better annuals that fills the bill․
What is different about this plant compared to Garden Phlox?
Tall annual phlox is closely related to the garden phlox (Phlox paniculata)‚ which forms a perennial mound and comes back each year‚ but in the humid Tennessee climate‚ garden phlox is often afflicted by powdery mildew to a degree that can ruin the ornamental appearance of the flowers․ Tall annual phlox must be planted each year‚ but is generally less susceptible to fungal disease than the perennial garden phlox․ People who have been forced out of perennial garden phlox by mildew have often grown the annual․
Colors and Bloom Characteristics
On each plant‚ the panicle (flower cluster) is about a dozen or more flowers on the end of each stem․ The small plant is almost completely covered by the large panicle of flowers‚ which can be blush pink or lavender or a mostly random mix of crimson‚ salmon colored and white on one plant․ Most have a different "eye" in the center․ The grandiflora series flowers are larger․ The Cecily series are more uniform and slightly smaller in stature‚ so they may be less desirable when planting in formal beds versus cottage beds․
Tennessee's zones and harvest dates
Tall annual phlox is a bedding plant and border annual that is hardy in USDA plant hardiness zones 2 through 11‚ which cover all of Tennessee‚ from the mountainous area around Johnson City in the east to the flatlands around Memphis in the west․ Hardiness zone is less important than frost date‚ because tall annual phlox is usually planted as an annual․
Since the last frost in Middle Tennessee doesn't occur until about mid-April‚ seeds are started indoors with grow lights 6-8 weeks before the last frost in early to mid-February․ As phlox seeds need light to germinate‚ they will not germinate properly if planted too deeply․ Gardeners in the higher elevations in East Tennessee may want to hold plants one more week or two before putting in the garden‚ although gardeners in slightly milder West Tennessee can direct seed by late March․
If you do not want to start phlox from seed‚ TN Nursery sells phlox starts you can plant in the garden․ Since the plants do not need to spend six weeks indoors sprouting‚ they bloom sooner and can also be planted later than you could have started from seed․
Site Selection and Soil
For maximum flowering‚ phlox need full sun (at least six hours of sun a day)․ However‚ by July and August‚ for phlox planted in full afternoon sun in the Nashville basin or Memphis area (especially in the hot and dry summer) some afternoon shade may prove helpful․ Plants do best in the morning sun with airy afternoon shade‚ which ensures healthier‚ more balanced flowering over the summer and into the late summer‚ than baking in the sun from noon to 6 PM․
Good drainage is necessary‚ as the soil needs to be moist without losing water to the roots between watering sessions․ Clay-rich soil in middle Tennessee needs to be improved with soil amendments before the trees can be planted․ Usually‚ if you can work in 2 or 3 inches of compost or well-rotted manure‚ it will solve most of the problems gardeners think are caused by overwatering that are actually caused by poor drainage․
Planting and Spacing
Plant the phlox 12 to 18 in (300 to 460 mm) apart․ Although the plants can be spaced closer together to create a fuller plant more quickly‚ this should be avoided since it decreases the amount of air circulation and increases fungal disease․ Just give them space and they'll fill in on their own within 4 to 6 weeks․
Wait to transplant seedlings until night temperatures are consistently over 45 degrees Fahrenheit (7 degrees Celsius) and‚ in Tennessee‚ frost occurs most years the last week of April and can kill young transplants․
Watering and Ongoing Care
It's best to water deeply once or twice a week rather than every day‚ to encourage deep roots that will be more drought-resistant as the plants mature․ During Tennessee's dry July and August months‚ full sun and container plantings may need to be watered three times a week․
A 2 in (5․1 cm) layer of mulch‚ such as the pine straw or shredded bark‚ can help to keep the roots of the plant cool and to reduce the number of times you have to water (by holding moisture and suppressing weeds)․
You need to deadhead every week or two through the bloom period‚ as most phloxes will dramatically slow their bloom rate if not deadheaded and allowed to self-seed by mid-summer․ Snip spent flower clusters just above the immediate next set of leaves․
A slow-release 10-10-10 or other balanced fertilizer may be used at planting time․ An application may also be made in mid-summer if the bloom set fewer flowers․ Avoid high-nitrogen fertilizers‚ which promote the green foliage and stems at the expense of flowers․
Surviving the Tennessee Summer Heat
Most gardeners find their phlox rather leggy and unattractive by late July․ At this time‚ plants can be cut back by one third to reinvigorate growth․ New growth will appear in 2 to 3 weeks‚ and the plants will usually bloom until the first frost of the season‚ which varies from mid to late October in much of Tennessee․
Staking and Wind Protection
While some of the larger cultivars can exceed 30 inches (760 mm)‚ which would cause the blooms to flop over during Tennessee's afternoon thunderstorms‚ a simple grow through stake or a few bamboo canes with twines can support them․ If wind is an issue‚ another method is to plant next to a fence or in front of taller shrubs for partial shelter․
Companion Planting Ideas
Tall annual phlox is often grown with zinnias‚ cosmos or marigolds‚ as they have similar growing requirements and make it easier to work the beds․ It can also be grown in a cutting garden‚ alongside snapdragons‚ which will give a variety of heights and textures to the planting․ In the perennial border the phlox fills the gap between the die-back of the spring flowering foliage and the emergence of the summer perennials and later flowering herbaceous plants and the gaps left by the later tulips․
You can seed them this February‚ or buy flat after flat of tall annual phlox and have months of color with no trouble‚ just what the Tennessee garden needs about this time․
