Giving your tubers the best chance of success
When growing for cut flowers, dahlias are certainly not a ‘set and forget’ plant. In order to get healthy growth, high quality flowers and useable tubers, plants need to be monitored and cared for throughout the season so that their needs are met at different stages.
Below are some care tips I have learned over the last few years - many of these learned the hard way through a fair bit of failure in my first couple of years! I also highly recommend this tuber guide by Summers Dream Farm, with really great examples of what makes a ‘good’ tuber and in what condition they are likely or unlikely to grow happily. This is a really handy guide for people who are buying or splitting tubers for the first time and wondering if their tubers look normal and viable.
Planting
Prepare beds with well rotted compost and cow or sheep manure, ideally. Horse manure should only be used if the horse is yours or you know the owner and can ask whether broadleaf weed sprays have been used around the horse paddock. If you don’t know this, you risk introducing aminopyralid into your patch which causes deformities to dahlias. Chicken manure can be used in moderation, but it is higher in nitrogen than cow and sheep and can result in too much leafy growth at the expense of flowers.
Decide how you will support the plants. The most successful ways to support dahlias are individually staking each plant using a thick, tall stake (which you should insert into the soil at time of planting); use a lesser number of stakes around the bed and ‘corral’ the plants using jute; or use plastic or jute netting stretched over strong stakes or star pickets. With this method, the dahlias grow up through the netting and the tension between the stakes/pickets keeps the plants upright.
Plants should be spaced at least 30cm apart, but ideally more if you aren’t regularly cutting from them or are in a high-humidity area. Here in Northern NSW I plant about 40cm apart, sometimes a little more for large cultivars, as I want to balance getting lots of plants in the space I have with the risk of powdery mildew spreading if they are too close.
If you are growing in pots, select a pot at least 30cm wide (but ideally bigger) with lots of holes for drainage. To improve drainage you can also add a layer of small stones to the bottom of the pot. Drainage is very important in pots as tubers don’t enjoy sitting in wet potting mix. Avoid using un-glazed terracotta pots, as they absorb the moisture and leave the potting mix too dry.
When digging a hole, make sure the soil is fairly loose and fluffy and incorporate more compost in the hole if needed - this is especially important if you have clay-based soil. The hole should be about 15-20cm deep. Lay the tuber in the hole horizontally, with the eye (if visible) facing upwards. If you can’t see the eye, don’t worry - it will find it’s way upwards!
Fill in the hole with your soil - you can add more compost here too. At this point I mix in some slow release fertiliser pellets to the soil too.
It is beneficial if the soil is a little damp when you plant, as this will help to wake up any tubers that haven’t yet sprouted. If your soil is completely dry, and you have no rain on the forecast for the coming week, you can water in lightly.
Mulch well to keep the soil moist and reduce weeds. I use sugar cane because it is easily available and affordable in my area.
Growing
Unless your soil is very dry, there’s no need to water again until the shoot is several inches big.
When the stem is 20-30cm in height and has several sets of leaves, you can pinch out the tip (down to the top set of leaves) to encourage lateral growth and more stems.
Dahlias are thirsty and like most perennials they prefer deep watering or very regular lighter watering so they get a good drink. The late John Menzel of Winkie Dahlias recommended pulse watering for dahlias, as it maintains a consistent horizontal damp area while being extremely water-wise. You can learn more about this approach here.
Especially if you are growing in pots, be very careful to not overwater. Drainage can be poor in pots, so it is helpful to use a water meter (about $15 from Bunnings) to check whether watering is needed.
If you have used pellet fertiliser, you can reapply after around 6 weeks. Once you start to see buds forming, stop using pellets/blood and bone and instead switch to potash. This can be bought in liquid or granular form, and promotes quality flowers rather than leafy growth. Once we get to mid autumn, feeding can stop - too many nutrients can soften tubers and lower their changes of surviving storage.
Foliar feeding can also be a great way to keep plants healthy and is an opportunity for you to inspect plants for problems too. Ideally foliar feeding should happen in the evening in summer, to avoid leaf burn.
Troubleshooting pests, deficiencies and disease
Floppy leaves in the day that return to normal in the evening: Transpiration. This is completely normal behaviour on hot days - don’t be tempted to water each time you see it happening though, as you will risk rotting the tuber. If the leaves are still floppy at night, dig down a little to check on the tuber, as this can be an indication of rot.
Leaves curling under, red spots on the underside of leaves, webbing on the underside of leaves, brittle curled leaves: Mites. The most common are red spider mites which thrive in humid conditions. Blast them off with a hose and either apply a miticide (select one depending on whether you grow organically or not) or purchase predator bugs to manage the population in the long term. For a large number of dahlias this is my preferred approach.
Deformed new growth, rubbery leaves that turn brittle and hard, no buds forming: Likely herbicide damage (such as aminopyralid) from contaminated manure/compost or spray drift from a nearby garden or farm. Cut the plant back to 2 or 3 sets of leaves, give it a feed and ensure it is regularly watered to flush out the contamination, and continue cutting back until new growth has returned to normal. Your tubers won’t be affected.
Yellow, ringed spots forming on leaves, yellow patterns on leaves, leaf mottling: Possible virus. This can be difficult to diagnose, as often virus looks like thrip damage. The American Dahlia Society has a great photo reference guide about virus symptoms here. Tubers with virus should be burned or thrown in the rubbish, not composted. Disinfect any tools that have come into contact with the plant. It can be tempting to hang onto the plant as often they still flower nicely, but you will risk infecting your other plants and suffering losses later down the track.
White powdery substance over leaves: Powdery mildew. This is usually caused by too much humidity from overhead watering or rain events in hot summers. Some people swear by worm sprays to reduce the spread and others use fungicides (eco-fungicide is available too), but the best way to avoid it is to not water from overhead and not plant too close - there needs to be enough air able to circulate through them to avoid a little humid microclimate. Stripping the lower plant leaves also helps this.
Random discolouration, flecks and markings on the underside of leaves: Probably thrips and/or aphids. Although tiny, they can do a lot of damage to plants and deform the flowers. You can use sprays to manage, and beneficial bugs. The best defence for thrips and aphids is a health ecosystem: encouraging small birds into your growing area, not using chemical sprays so ladybugs can thrive, growing a wide variety of plants for pollinators, and doing pH and soil tests so you can identify imbalances that make plants more at risk of damage.
Loads of shoots emerging from the tuber, that mostly don’t progress into stems: Leafy gall. It is common for one tuber to have several shoots or for a small clump to have shoots coming from each of the tiny tubers. With gall you will find an un-countable number of usually white stunted shoots clustered together, sometimes making it look a bit like a yucky cauliflower. Tubers with gall should be destroyed and tools disinfected. Like with virus, plants with gall can often flower nicely but you should not risk leaving them to grow as it will spread to other plants.
Dahlias make us work hard but if you have a plan for caring for them, they will absolutely make it worth your while!