🌿 Dahlia

Dahlias thrive in fertile, humus-rich, well-drained soil that retains some moisture but never remains soggy. A potting soil amended with plenty of compost or well-decomposed manure is ideal; improve heavy clay with organic matter and coarse sand (or plant in raised beds) to increase aeration and drainage. In sandy soils, add organic matter to increase water and nutrient retention. Aim for a slightly acidic to neutral pH (around 6.0-7.0). In containers, use a mixture of draining potting soil with added compost and perlite/grit, and make sure there are good drainage holes.

Dahlias are best propagated vegetatively (tubers or cuttings). Seeding is possible but not commonly used, as seedlings are not true to type and results are highly variable. From tubers (recommended) - Timing: Plant after all danger of frost has passed and the soil has warmed up; dahlias are sensitive to frost. In cool climates, start potted tubers indoors a few weeks before the last frost, then harden off and plant out once the nights are mild. - Location/soil: Full sun; fertile, well-drained soil. Avoid cold, waterlogged soil. - Planting depth: Place tuber horizontally, eye or crown just below soil surface; cover so that crown is about 5-10 cm deep (deeper in very sandy soils, shallower in heavy soils). - Spacing: Generally 45-60 cm for small types and 60-90 cm for large types. - Watering: Water lightly after planting if the soil is dry; thereafter, maintain regular but not saturated humidity. Avoid heavy watering until the shoots have emerged, to reduce the risk of rotting. - Support: Insert stakes at planting time for tall varieties to avoid damaging tubers later on. Division (to increase stock) - Divide dormant clumps so that each division includes a part of the crown with at least one visible eye (bud). Plant the divisions as described above. Cutting (for uniform plants) - In late winter or early spring, place preserved tubers in a warm, light environment to germinate. When the shoots reach around 7-10 cm, take basal cuttings with a heel, root them in a non-draining medium with high heat and humidity, then pot and plant them after the frosts. From seed (for mixed varieties) - Sow indoors 6-8 weeks before last frost at warm temperatures (approx. 20-24°C / 68-75°F) in seed potting soil; cover lightly and provide bright light. Transplant when large enough to handle and grow frost-free. Harden off and transplant outdoors after frosts. Plants grown from seed may form small tubers at the end of the season.

Dahlias feed moderately and perform best with a regular diet that is not rich in nitrogen. Incorporate compost or well-decomposed manure into the bed before planting; a soil test is ideal to guide rates. When planting, use a low-nitrogen, high-phosphorus/potassium fertilizer (e.g. 5-10-10, 4-10-10 or similar) lightly incorporated into the soil around the tuber (not directly against it). Once the shoots have reached a height of 15-20 cm, start feeding them every 3-4 weeks with a low-nitrogen granular fertilizer or dilute liquid fertilizer; avoid high-nitrogen lawn fertilizers and high-nitrogen blends, which promote soft, leafy growth and fewer flowers. Continue fertilizing regularly until maximum flowering, then reduce or stop fertilizing in late summer or early autumn (about 4-6 weeks before the expected frost) to help tubers mature. For containers, use a balanced liquid flowering fertilizer in half doses every 1-2 weeks, rinsing occasionally with ordinary water to avoid salt build-up.

Apply a 5-8 cm layer of organic mulch (compost, leaf mold, well-decomposed bark or straw) after planting, once the soil has warmed up, to conserve moisture, moderate soil temperature and eliminate weeds. Keep the mulch a few centimetres from the stems/crowns to reduce rot and slug problems. In summer, top up the mulch as it decomposes, but avoid mulching too thickly in poorly drained soils where tubers are likely to rot. In cold-winter climates, after frost has blackened the foliage, mulch heavily (10-15 cm / 4-6 in) over the crown for insulation if tubers are left in the ground, ensuring that the site is well drained.

When planting, plant stakes for dahlias, especially tall, large-flowered ones, to prevent wind damage and stem breakage. Use a sturdy cane, bamboo stake or metal rod (or a tomato-style support/cage for bushy plants) placed just outside the tuber/bud and press it in firmly. As the stems grow, tie them to the support at regular intervals with flexible ties (e.g. string, Velcro or plant ties) in loose figure-of-eight loops to avoid compressing the stem. Add additional ties higher up if necessary, keeping flower stems supported but not bent.

Pinch out young plants when they are 20 to 30 cm tall (after 3 to 4 pairs of leaves) to encourage branching and obtain a bushier plant. To prolong flowering, regularly cut off wilted flowers at a leaf joint or strong side shoot. For large, showy flowers, remove the small lateral buds and keep one main bud per stem; for more numerous but smaller flowers, leave the buds intact. In spring, thin out crowded shoots in the crown, keeping a few strong stems and eliminating weak shoots. After the first frost, which blackens the foliage, cut stems back to around 10-15 cm before lifting/storing tubers or mulching in mild climates; remove any soft or damaged stem tissue.