Learn how to grow dahlias. Always choose the right variety, plant in full sun, water regularly, fertilize monthly, and provide support.
How to Grow Dahlias?
Dahlias have become synonymous with summer. These beautiful blooms, known for their wide array of vibrant and soft pastel shades, can brighten any border from July through October – they add late summer color in any style garden, whether that means cottage-style borders or jungle or exotic schemes – as they look amazing alongside cosmos, grasses, Verbena bonariensis or cannas.
Dahlias come in various sizes. Dwarf varieties make suitable bedding plants while more compact varieties thrive in pots; tree dahlias (Dahlia imperialis), from which modern dahlias were developed, can reach 5m tall.
Dahlias make beautiful and prolific cut flowers; as soon as you start cutting them, more blooms appear! However, due to their complex flowers being unattractive to pollinators such as bees and butterflies. Be sure to include single-flowered varieties if possible – these are preferred by pollinators.
Dahlias originate from Central America; brought over by the Spanish for consumption initially. They require ample sunlight to flourish and should be protected during colder weather periods for best results.
Where to Grow Dahlias?
Dahlias thrive in full sun – south or west-facing borders are ideal – and don’t require special soil conditions; any fertile, moist yet well-drained fertile soil with some organic matter such as well-rotted manure is sufficient. For optimal results when growing dahlias in pots or garden beds use high-quality, peat-free multipurpose compost.
Growing Dahlias From Tubers
Dahlia tubers can be purchased beginning in February; most garden centers carry an excellent selection, but for the widest selection shop online.
Early April can be the ideal time to organize and store all of your tubers, lightly covered in compost. Or you could pot each tuber up individually in a plastic pot filled with multi-purpose potting compost – positioning each one correctly so it sits just below the soil surface and water well so any excess can drain away.
Install the tubers in a warm, frost-free location (such as a greenhouse, cold frame, porch, or windowsill) and maintain a moist compost. New shoots should appear about five weeks post-planting and have formed bushy plants by May when the frost has finished its cycle.
How to Plant Dahlias?
Dahlias can be grown easily with support. Here are the steps for planting them successfully:
Dahlias Can Be Planted Out After no frosts are forecast in May or later, dahlias may be planted out into your garden. Before doing so, some shoots from each tuber should be removed to encourage bushier plants with more flowers – cutting back any extra shoots can also make basal cuttings as seen below.
Harden off your plants (acclimatize them to outdoor conditions) before planting your dahlias outside – place them outside during the day but bring them inside at night for at least a week or more until planting time arrives.
Dig a planting hole about 30cm across, adding one bucketful of well-rotted organic matter for increased soil fertility. If your soil is heavy clay, consider adding some horticultural grit as drainage to aid drainage of the planting hole and protect new shoots from being eaten by snails and slugs. When planting from garden center pots, do so at their original depth to maintain consistent success in growth.
Dahlias require plenty of room to grow – aim to plant them 60cm apart for maximum impact
Once your dahlia is planted, it needs some support unless it’s a dwarf variety. Dahlias grow quickly and are susceptible to flopping over when flower heads are large, so add sturdy stakes or bamboo canes at all four corners (these will become part of its structure as the plant matures), then tie any new growth every couple of weeks for optimal success.
If planting dahlias in a pot, select one at least 30cm wide and deep before filling it with multipurpose compost mixed with organic matter such as well-rotted manure and adding slow-release plant food at the same time. Be sure to plant at the same depth they were in their original pot.
How to Care for Dahlias
Once your Dahlia plant reaches about 20cm in height, pinch out its main growing tip to encourage more flowering side shoots by pinching out. Do this using either a sharp knife or your thumb and forefinger as this will encourage new side shoots of blossoming flowers to form from further branches of growth.
Add new growth to the stake every few days or so.
Deadheading dahlias to extend flowering is essential. While distinguishing between buds and spent flower heads may be tricky – buds tend to be round while spent ones can have more pointed petals – here, Monty provides instructions for deadheading dahlias:
If you are cultivating dahlias for cutting purposes, cut their flowers when they have opened fully.
Overwintering Dahlias:
Overwintering Dahlias Dahlias will not survive overwinter in many areas if temperatures are very frosty or wet, so it is wise to lift their tubers once their foliage has become blackened by frost, cut back the stems to about 12cm, gently lift and remove soil with fingers before placing upright into a newspaper-lined tray so they may dry for two or three weeks in a dry environment.
Once dry, store tubers in a cool and frost-free location in a shallow tray of compost or horticultural sand without watering; they don’t require light during this period – making a dark garage ideal for storage. They should remain here until late March or early spring when you plan on repotting.
If you live in a mild area with well-drained soil and don’t have the space for lifting and storing dahlia tubers, or don’t want the hassle of lifting and storing dahlias each winter, or have no room to lift and store tubers during frost season, cover the soil with a thick layer of mushroom compost or straw to protect your tubers against frost damage and remove in spring once this layer has decomposed. Simply take away any extra compost.
Propagating Dahlias/Propagating Dahlias by Cuttings
Dahlia tubers of sufficient size may be divided in the spring by pressing them down onto a tray of shallow compost and leaving them for growth in a greenhouse or sunny room. When shoots appear, divide using a sharp knife so each section contains at least one shoot and set of roots; pot these divisions and plant out once all danger of frost has passed.
Dahlias can also be easily propagated by taking basal cuttings in spring from each tuber. Each cutting will produce five new plants which will bloom this summer. You can also purchase online rooted cuttings at this time; these may produce smaller plants initially so are best suited for pots but will still flower beautifully.
Growing Dahlias: Solve Your Problems
Dahlias can be particularly susceptible to attacks by earwigs, which feed off of their foliage, buds, and flowers. To create effective traps for these unwanted visitors use garden canes pushed into the soil with an upturned pot filled with straw placed overtop of it; when day breaks they’ll head for that pot instead and you can dispose of them once their mealtime has come and gone.
Slugs and snails love dahlias, especially their new growth. Planting out plants when they have reached an acceptable size helps – as does surrounding the plant with a copper ring – while organic slug pellets won’t harm wildlife; read our guide on protecting young plants from slugs for more tips.
Capsid bugs are another potential problem, and in our Quick Tips video, you’ll learn how to protect dahlias from being devoured by them.
FAQs
Q1. Dahlias grow better in pots or the ground?
Dahlias can grow better in the ground where they have more space.
Q2. What month do you plant dahlia tubers?
You should plant dahlia tubers in the spring.