Dahlia Care in August: What to Do at Peak Bloom

August is not actually the tuber month

The common framing is that August is when tubers start bulking up and everything you do should serve that. The research points somewhere slightly different, and it is worth understanding because it changes what this month is for.

Tuberous root development in dahlias is triggered by short days. Moser and Hess put the critical daylength at roughly 11 to 12 hours, below which the plant shifts from building shoots to storing reserves. Legnani and Miller demonstrated the mechanism directly: under long days, tuberous root development was inhibited while shoot dry weight rose, and under short days the tuberous root tissue accumulated 156 percent more fructan, the storage carbohydrate. Total plant dry weight did not change. The plant is not making more, it is deciding where to put it.

Now do the calendar arithmetic. At around 40 degrees north, daylength is still roughly 14 hours in mid-August and does not reach 12 hours until the September equinox, or drop below 11 until well into October. So the heavy bulking phase is a late-September-into-October event. August is still mostly flower mode.

That reframes the job. August is not when you fill the tubers. It is when you protect the leaf area and plant health that will fill them six to ten weeks from now. Every decision below follows from that.

Two honest caveats on the science. Legnani and Miller worked with seedlings rather than named cultivars grown from tubers, and the more recent photoperiod work used a dahlia species rather than a garden hybrid. The direction of the effect is well supported; the exact numbers for your particular cultivar are not.

The feeding question, and where the advice actually comes from

You will read everywhere that you should stop feeding dahlias in August, or thirty days before your first frost. We went looking for the source of that and the picture is messier than the confidence suggests.

What is documented is the mechanism, not the date. Oregon State Extension's dahlia bulletin states that if too much nitrogen is applied, the result is weak growth, late blooming, and poor keeping quality of the tubers. That is a real, sourced consequence, and it is about how much nitrogen, not what month you last applied it.

The calendar cutoffs come from elsewhere. Chicago Botanic Garden says to feed with a 5-10-10 or 5-20-20 mix and stop in mid-August, which is explicit but comes with no stated reasoning. The American Dahlia Society describes the pattern of high nitrogen early and minimal nitrogen late as traditional wisdom, which is the society's own word for it, not ours. The widely repeated thirty-days-before-frost rule we could not trace to any extension service, dahlia society, or botanical garden at all.

The ratios are also less settled than they look. Oregon State's actual recommendation is 5 or 6 pounds of a 0-20-20 fertilizer per 100 square feet worked in at planting, with nitrogen applied later only if it is needed, as the buds form. Chicago Botanic Garden's 5-10-10 or 5-20-20 is a different program. Both are low-nitrogen relative to a general-purpose feed, and that is the point they agree on.

So the defensible version for August: if you are still feeding, use something low in nitrogen, and do not push growth now. The reason is not that a date has passed. It is that surplus nitrogen buys you soft late growth at the cost of how well the tubers keep, and you are about to ask those tubers to survive five months in a box.

Humid August, and the job that does double duty

August weather in much of the country is warm, humid, and increasingly crowded in the border. Two fungal problems track that, and they behave differently enough to be worth separating.

Botrytis, gray mold, is the one that actually connects to a job you are already doing. The Pacific Northwest Pest Management Handbook notes that Botrytis cinerea builds rapidly on soft, succulent material during cloudy weather and high humidity, and specifically that it colonizes and sporulates readily on faded dahlia flowers, with senescent leaves colonized too. That makes deadheading a disease-control task in August, not only a bloom-production one. A spent bloom left on the plant in humid weather is an inoculum source sitting in the middle of the canopy.

It also does not stop at the end of the season. The same handbook describes Botrytis following tubers into storage, where infected tissue turns moist and water-soaked and develops gray spore masses. Sloppy August sanitation can show up in a January storage crate.

Powdery mildew is the more visible problem and, in the United States, apparently the smaller one. The causal fungus on dahlia is Golovinomyces cichoracearum, and the handbook's own assessment is that it is not considered a large problem in the US, though it has been troublesome elsewhere. Its conditions are counterintuitive: unlike most fungal diseases, it is favored by high humidity with dry leaves, so it is not something you prevent by keeping foliage dry.

One thing we are not going to tell you is whether late-season powdery mildew on dahlias is worth treating. You will read that late-season mildew is cosmetic, and that reasoning is real, but it comes from extension guidance about trees and shrubs, whose leaves have nearly finished their work and are about to drop. Dahlia foliage in late August has not finished its work. It is the engine for the tuber bulking that has not happened yet. We found no research linking powdery mildew severity on dahlias to tuber yield, so the honest answer is that nobody has established it either way. What is uncontroversial is that fungicides applied after symptoms appear do little, and that airflow through the plant helps, which is one of the reasons the ADS recommends opening plants up when disbranching.

Decide now which plants you will keep

This is the most August-specific job on the list, and the one most likely to be done badly, because the obvious method does not work.

Dahlia viruses are common, there is no cure, and infected stock stays infected. Utah State Extension is blunt about it: there are no resistant varieties and plants cannot be cured once infected, so the guidance is to destroy symptomatic plants and not propagate them. If you are going to lift and divide in October, the selection decision is effectively made now while the plants are still standing and you can tell them apart.

The problem is that looking at them is not enough. The American Dahlia Society's testing work reports that the RNA viruses in dahlias can all exist in plants that are completely asymptomatic. So a plant that looks fine may not be, and visual roguing catches only part of the problem. That is not a reason to skip it, but it is a reason not to trust it as a clean bill of health. Our guide to what virus-free actually means, linked below, covers the testing and sanitation side in full; what follows here is only the part that is specific to deciding in August.

The ADS work also found that not all of these viruses behave the same way across seasons. Tobacco streak is almost always retained into the next year. Tomato spotted wilt is frequently present in the following generation. Impatiens necrotic spot is almost always absent in the next generation, to the point that ADS suggests there may be minimal risk in continuing to grow a plant that tested positive for it. So a positive result is not automatically a death sentence for a cultivar you value.

On prevalence, we are going to decline to give you a single number. The ADS position has moved: a 2022 bulletin described dahlia mosaic and its relatives as virtually absent from recent test results, while the 2025 update reported them widespread in almost all gardens in the 2023 season, with nearly 70 percent of plants positive in one garden. Open testing was suspended in 2024. Anyone quoting you a confident single percentage is quoting something the society itself has revised.

The practical August habits that follow are small and they matter. Mark the plants you intend to keep now, while you can still see which is which. Pull and destroy anything with clear virus symptoms rather than composting it. And disinfect your cutting tools between plants, because ADS field work found that a single cut with un-disinfected cutters transmitted virus to a clean plant, with published work suggesting one contaminated cut can carry to a long run of subsequent plants. If you are cutting armfuls of dahlias in August, you are also potentially moving virus around the patch all month.

Keep cutting, and what nobody actually knows

Cut hard in August. The plants are at their most productive and cutting is what keeps them producing.

The question people ask at this point is whether heavy cutting costs them tuber size, on the reasoning that the plant only has so much to give. It is a sensible question and we could not find an answer to it. There is no study, extension publication, or dahlia society document we could locate that tests whether cutting blooms affects tuber yield.

There is an argument in each direction, which is usually the sign that the honest answer is unknown. Removing flowers removes a competing sink for the plant's resources, which would favor the tubers. But cutting a long dahlia stem also removes a lot of leaves, and leaves are what make the carbohydrate the tubers store, which cuts the other way. The ADS grooming instructions include a pointed reminder not to remove the leaves when disbudding, which suggests the people who do this competitively take leaf retention seriously.

So cut your flowers. That is what you grew them for. Just be aware that anyone telling you confidently that hard cutting either does or does not shrink your tubers is extrapolating, not citing.

Setting up for the lift without getting ahead of it

Nothing gets dug in August, but a few things are worth knowing now so the end of the season does not catch you improvising.

The trigger is well established. New Hampshire Extension advises waiting to dig until the top growth dies back or is killed by the first hard frost, and Chicago Botanic Garden says after several hard frosts have blackened the plants. The reason to wait is equally well supported and is the same photoperiod story from the top of this guide: tubers keep developing after the foliage is gone, and ADS notes that the longer they cure in the ground, the more fully developed they are and the better they keep.

Two pieces of common advice around this are weaker than they sound. The specific instruction to wait ten to fourteen days after the first killing frost we could not source to any extension service, dahlia society, or botanical garden. And the advice to taper watering in the last month before frost we could not source either. Neither is likely to hurt you; both are convention presented as protocol.

It is also not a rule that you must wait for frost at all. ADS explicitly allows digging before frost, particularly where a wet autumn is the real threat, while still advising small growers to leave roots in the ground as long as is practical. Heavy clay, a forecast hard freeze, or a Pacific Northwest rainy season are all legitimate reasons to go early.

The one practical August note about the lift is about staking. Every source that addresses support says to stake early, when the plant is around a foot tall or at planting. There is no authoritative late-season staking guidance, and the reason to stake early is fairly obvious once you are standing in an August border: driving a stake in now means driving it into tubers that are already there. If a plant is leaning and you have to intervene, tie it to something already in the ground rather than adding a new stake.