August is when my dahlias either reward me for all my spring optimism or remind me, very plainly, that gorgeous plants still need disciplined care. If you’re growing dinner plate dahlias and hoping for those enormous, theatrical blooms before frost, this is the month that matters most. In my Midwestern garden, August can bring steamy days, pounding storms, dry spells, and cool nights all in the same two weeks, and dahlias respond to every bit of it.
The good news is that a few timely jobs can make an astonishing difference in bloom size, stem strength, and how many flowers actually open before the season ends. Below, I’m walking through the August tasks I never skip, from feeding and watering to disbudding, staking, and keeping disease pressure low. If you want those big, florist-worthy flowers that stop people in their tracks, this is the checklist I’d follow starting today.
1. Water deeply and on a schedule, not by guesswork
Dinner plate dahlias cannot build massive blooms on erratic moisture. In August, I aim for about 1 to 1.5 inches of water per week from rain or irrigation, and during stretches above 88°F, I often increase that to closer to 2 inches. That usually means a deep soak 2 or 3 times per week rather than a light daily sprinkle. A shallow watering encourages shallow roots, and shallow roots are the first to suffer when the weather swings hot and windy.
I water at the base of the plant, ideally in the morning between 6 and 9 a.m. If I’m using a hose, I let it run slowly for 20 to 30 minutes per clump. If I’m on drip irrigation, I’m looking for moisture 6 to 8 inches down in the soil. A simple test is to stick your finger or a trowel into the soil: if it’s dry below the top 2 inches, it’s time to water. Wet foliage late in the day is an invitation to powdery mildew and leaf spot, especially in humid late summer weather.
2. Feed for blooms, not leafy growth
By August, I stop pushing dahlias with high-nitrogen fertilizer. Too much nitrogen this late gives you lush leaves and thick stems, but fewer blooms and softer growth that can flop or attract pests. What I want now is support for flowering, so I switch to a low-nitrogen, higher-phosphorus and potassium product such as a 5-10-10 or 0-10-10 bloom fertilizer.
I usually apply according to label directions, but as a rough example, granular fertilizer often runs about 2 to 3 tablespoons per plant, scratched lightly into the soil 6 inches away from the main stem, then watered in well. If I’m using a liquid feed, I dilute exactly as directed and apply every 10 to 14 days through late August or very early September. In rich garden soil, one August feeding may be plenty. In containers, plants usually need more frequent feeding because nutrients wash out faster.
3. Disbud if you want truly giant dinner plate flowers
This is the step many gardeners skip because it feels almost cruel, but it works. If your goal is the biggest possible bloom, remove the smaller side buds and keep the central terminal bud on each flowering stem. That concentrates the plant’s energy into fewer flowers, and the remaining blooms can become dramatically larger.
On a typical dahlia stem, you’ll often see one main bud in the center and 2 smaller side buds just below it. I pinch off those side buds when they’re still small, about the size of a pea to a marble. For exhibition-sized flowers, I may also limit the number of flowering stems per plant, especially on younger or less vigorous plants. It feels like less at first, but by late August and September, the payoff is obvious: larger flower diameter, better form, and stronger stems for cutting.
4. Keep deadheading relentlessly
Dahlias are bloom machines, but they perform best when spent flowers are removed promptly. In August, I deadhead every 2 to 4 days. If you leave old blooms on the plant, it starts diverting energy into seed production instead of making new flowers. That’s the opposite of what we want with frost on the horizon.
Make your cut just above a strong leaf node or lateral branch, not right under the old flower head. That encourages the next flowering stems to develop lower on the plant, where stems are usually sturdier. One little trick I learned after years of squinting in the garden: spent blooms look pointed or cone-shaped, while fresh buds look rounder. Once you learn that difference, you can deadhead much faster and avoid cutting promising flowers by mistake.
5. Stake and tie before a thunderstorm does the pruning for you
August is peak storm season where I live, and a fully loaded dinner plate dahlia can snap overnight if it isn’t supported. Large blooms act like sails in wind and like sponges in rain. A single stem with a 9- to 11-inch flower can get surprisingly heavy, especially after a storm.
I prefer sturdy stakes at least 5 to 6 feet tall, driven 10 to 12 inches into the ground. Soft plant ties, jute twine, or stretchy garden tape work well. Tie stems loosely in a figure-eight pattern so the plant has support without being strangled. For especially vigorous varieties, I’ll add 2 or 3 tie points as the plant grows, spacing them about every 12 to 18 inches up the stem. If your plants are already leaning, correct them gradually over several days rather than wrenching them upright all at once.
6. Mulch to hold moisture and steady soil temperature
One of the simplest August upgrades is a 2- to 3-inch layer of mulch around each plant. I like shredded leaves, clean straw, or finely shredded bark. Mulch reduces evaporation, keeps the root zone cooler during heat spikes, and cuts down on soil splash, which can spread fungal problems onto lower leaves.
Keep mulch 2 to 3 inches away from the main stem so you don’t trap moisture directly against the crown. If your garden soil is heavy clay, don’t pile mulch too thickly after a rainy stretch; you want consistent moisture, not soggy conditions. In containers, mulch helps too, though I use a thinner layer, about 1 inch, so pots don’t stay overly wet.
7. Remove lower leaves to improve airflow and reduce disease
By August, many dahlia plants are dense enough that air barely moves through them. That’s a setup for powdery mildew, botrytis, and general leaf decline. I remove the lowest 8 to 12 inches of foliage on mature plants, especially any yellowing, spotted, or crowded leaves. This simple cleanup improves air circulation and helps the base of the plant dry faster after watering or rain.
Always use clean pruners or snips. I wipe blades with 70% isopropyl alcohol between sick and healthy plants if I’m seeing disease. Don’t remove more than about one-third of the total foliage at one time, or you can stress the plant. The goal is not to strip it bare, just to open it up enough that light and air can move through the canopy.
8. Watch for spider mites, thrips, earwigs, and Japanese beetles
Late summer pests can ruin flowers just as they’re about to peak. Spider mites are especially common in hot, dry weather and often show up as pale stippling on leaves, fine webbing, and tired-looking plants. Thrips can scar petals and distort blooms. Earwigs chew flowers overnight, and Japanese beetles can skeletonize leaves and shred petals in a hurry.
I inspect dahlias at least twice a week in August, checking the undersides of leaves and looking into flower centers. A strong spray of water can knock back spider mites if you do it consistently every few days. Insecticidal soap can help on soft-bodied pests, but it must contact them directly and should not be sprayed during the hottest part of the day. I handpick Japanese beetles into soapy water in the morning when they’re sluggish. For earwigs, simple rolled newspaper traps or shallow containers with a little oil and soy sauce can reduce damage. The key is early action before populations explode.
9. Cut some flowers regularly to stimulate more blooms
It sounds backward, but harvesting your dahlias actually improves the display. Frequent cutting encourages the plant to keep producing new stems and flowers instead of slowing down. In August, I cut blooms early in the morning when stems are well hydrated, and I choose flowers that are nearly fully open because dahlias do not continue opening much after harvest.
Use sharp, clean pruners and cut deep into the plant, taking a long stem back to a leaf joint. If you only snip the flower head, you’ll end up with awkward stubs and weaker regrowth. I place stems immediately into warm water, not cold, and re-cut the ends before arranging. Many dinner plate varieties last 4 to 6 days in a vase, sometimes longer if harvested at the right stage and kept out of direct sun.
10. Stop wasting energy on weak stems and tiny late buds
As the season narrows, not every stem deserves to stay. If a stem is spindly, badly shaded, broken, or producing tiny buds that are unlikely to mature before frost, I remove it. This is especially useful in the second half of August in shorter-season regions. The plant has a limited amount of energy left, and I’d rather direct it into strong stems that can actually produce dramatic flowers.
On crowded plants, I may thin a few interior stems to improve light penetration and reduce competition. I’m not talking about hacking the plant apart, just making strategic cuts. If frost is usually 45 to 60 days away in your area, be realistic about which buds can finish. Larger dinner plate types often need several weeks from bud development to full-size bloom, so prioritizing the best stems can make the difference between mediocre and magnificent.
11. Protect plants from heat stress during extreme August weather
Dahlias love warmth, but there’s a point where extreme heat slows them down. When temperatures climb above 92°F for multiple days, I often see smaller blooms, faded color, and droopy foliage by afternoon. The solution is not more fertilizer. It’s moisture management, mulch, and sometimes a bit of temporary shade.
If you’re in a brutally hot stretch, a piece of 30% shade cloth during the hottest part of the afternoon can help, especially for dark-flowered varieties that scorch more easily. I’ve also had success watering deeply in the morning and then checking plants again at dusk. If a container-grown dahlia is wilting daily, move it where it gets morning sun and a little protection after 3 p.m. Recovery is much faster when you reduce repeated heat stress.
12. Start planning now for frost, tubers, and the final flush of bloom
August may feel early to think about frost, but this is exactly when I start counting backward. In much of the Midwest, the first frost can arrive sometime between late September and late October, depending on the year and the microclimate. That means every August task should be aimed at maximizing the next 6 to 10 weeks.
Keep a simple garden notebook or phone note with variety names, bloom performance, and which plants deserve prime space next year. If a particular dinner plate dahlia is producing 10-inch flowers on strong 36-inch stems, I want to remember that. If another one mildews badly or makes lots of foliage but few blooms, I want to remember that too. Come digging time after frost, those notes are invaluable. And in the meantime, they help you focus your August effort where it will bring the biggest payoff.
13. Know what “show stopping” actually looks like for your variety
Not every large-flowered dahlia reaches the same size, and this matters because unrealistic expectations lead gardeners to overfeed or overwater. A true dinner plate dahlia often produces blooms in the 8- to 10-inch range, and exceptional blooms can reach 11 or even 12 inches under ideal conditions. But size also depends on cultivar genetics, plant age, spacing, soil fertility, and whether you disbud consistently.
I’ve grown varieties that were breathtaking at 8 inches because the color was luminous and the form was perfect. I’ve also grown bigger flowers that looked coarse because the stems were weak or the petals weathered quickly. So yes, chase size if that’s your joy, but also pay attention to stem strength, petal substance, and repeat bloom. In the garden and in a vase, those traits matter just as much as diameter.