How to Grow
Zinnia 'Giants of California' from Seed
The heirloom giant zinnia — Half-Hardy Annual H2; huge 12–15cm (5–6") double blooms; full colour range; 90–120cm; dating from 1919; outstanding cut flower; sow individually in modules March–May at 21–24°C (no root disturbance); pinch at 10–15cm; plant June in warm soil; water at base only; cut hard and often for more stems; July–October
Zinnia Giants of California is one of the most celebrated heirloom zinnia varieties in cultivation — a plant dating from 1919 that established the standard for large-flowered cutting zinnias and has never been displaced from that position. The flowers reach 12–15cm (5–6") across in the full zinnia colour range — red, orange, yellow, pink, lavender, purple, and white — on plants growing 90–120cm (3–4ft) tall, producing the long stems and dramatic bloom size that make this variety outstanding in the cutting garden and spectacular in large border plantings.
Three rules govern successful zinnia growing: sow individually (zinnias hate root disturbance — never in open seed trays), plant in genuinely warm soil in June (cold soil causes slow, sulky establishment), and always water at the base (overhead watering triggers powdery mildew). Observe these three rules and Giants of California responds with one of the most prolific and rewarding cut flower displays available from a UK summer garden — cutting hard and often producing more and more stems as the season progresses, right through to the first October frosts.
Quick Facts at a Glance
Plant Type
Half-Hardy Annual H2 — the heirloom giant zinnia; sow indoors in individual pots March–May
Flowers
Huge 12–15cm (5–6") double blooms in the full colour range; tall sturdy stems to 90–120cm
Heirloom
Dating from 1919; one of the most celebrated zinnia varieties; the classic exhibition cut flower
KEY rule
Sow individually (hate root disturbance); plant June in warm soil; always water at base
Cut-and-come-again
The more you cut, the more they produce — grows stronger with regular harvesting
Difficulty
2 out of 5 — easy with the right technique; the no-root-disturbance and warm-soil rules are key
Understanding Giants of California
Half-Hardy Annual — Sow Indoors; Hate Root Disturbance; Plant Warm Soil June
All zinnias are Half-Hardy Annuals (H2) from Mexico. They are heat-loving and frost-tender at every stage. Three specific requirements distinguish successful zinnia growing: sow in individual modules or pots (never in open trays) because zinnias hate root disturbance and suffer badly when transplanted from a communal tray; maintain warmth at 21–24°C for germination (5–7 days when warm); and plant out only in June when the soil is genuinely warm — cold soil causes slow sulky establishment. Zinnias are highly susceptible to powdery mildew — always water at the base, never overhead.
The Heirloom Giant — 1919 and Still Supreme
Zinnia Giants of California is one of the oldest and most celebrated zinnia varieties still in cultivation. Dating from 1919 (with some sources noting a related introduction by Bodger Seeds of California in 1926 that won an RHS Gold Medal), it established the standard for large-flowered zinnias that all subsequent breeding has sought to refine rather than replace. The flowers reach 12–15cm (5–6") across on plants growing 90–120cm (3–4ft) tall — a scale that makes them dramatic as garden plants and outstanding as cut flowers, with the long stems required for proper vase arrangements. The full colour range includes red, orange, yellow, lavender, purple, white, and deep pink.
Pinch for More Stems; Cut Hard for More Flowers
Pinching Giants of California when plants reach 10–15cm tall — removing the growing tip — produces a branching, multi-stemmed plant that generates many more cutting stems than an unpinched specimen. Once flowering begins, cut stems long (to just above a leaf node) rather than deadheading at the flower head alone. The harder and more frequently you cut from a zinnia, the more vigorously it responds with new stems. Plants that are cut regularly throughout the season continue producing right up to the first frost; plants that are left to set seed slow and stop.
Sowing & Growing On
Sow March–May Individually at 21–24°C — 5–7 Days — No Root Disturbance — Pinch at 10–15cm — Plant June in Warm Soil at 45cm — Water at Base Only — Cut Hard and Often
Sow March–May in individual modules at 21–24°C (5–7 days). No root disturbance at transplanting. Pinch at 10–15cm. Plant June in warm soil at 45cm. Water at base only. Cut hard and often for more stems.
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Sow March–May individually in modules or biodegradable pots at 5mm depth. Sow one seed per module, pointed end down, at 5mm depth. Maintain at 21–24°C. Germination typically 5–7 days at this temperature. Do not sow in open seed trays — individual sowing containers are essential because zinnias hate root disturbance at transplanting.
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Grow on at 18–20°C in the brightest available position; do not allow roots to circle the pot. Pot on into 9cm pots if the module becomes full before planting time. Zinnias grow fast once established — maintain them in active, well-lit growing conditions to prevent etiolation (leggy, drawn growth caused by insufficient light).
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Harden off over 7–10 days; plant out in June when the soil is genuinely warm. Space at 45cm. Giants of California grows large — give adequate space for air circulation and to prevent powdery mildew. Space according to variety. Plant carefully to avoid disturbing the root ball — slide gently from the module or pot without breaking the root system. Water in well after planting.
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Pinch out, deadhead consistently, and feed fortnightly from first flowering. Pinch out the growing tip at 10–15cm to produce a multi-stemmed branching plant. Without pinching, plants produce fewer, taller stems. Once flowering begins, cut stems long to just above a leaf node — the more you cut, the more the plant produces. Deadhead all spent flowers consistently — this is the most important ongoing care action for extended zinnia flowering. Feed every two weeks with a high-potash liquid fertiliser from the first buds. Water at the base only — overhead watering causes powdery mildew.
Growing On & Care
The Exhibition Cut Flower
Giants of California produces stems of 45–60cm or more bearing the enormous 12–15cm double blooms that make it the classic exhibition and florist's zinnia. Cut in the morning when flowers are at their freshest and the outer ring of petals is fully open. Vase life 7–10 days. The full colour mix — including the distinctive lavender and purple tones that are rarely found in other cut flowers of this season — makes Giants of California uniquely useful in mixed summer arrangements. The large, structured blooms provide focal points around which smaller-flowered companions arrange themselves naturally.
The Full Colour Range
Giants of California produces the full zinnia colour spectrum in a single packet: vivid red, warm orange, rich yellow, deep pink, soft lavender, mauve-purple, pure white, and blends in between. Because the plants are open-pollinated, the colour distribution within a planting is variable and unpredictable — one of the qualities that makes a mass planting of Giants of California so visually exciting. The lavender and purple tones are particularly valuable as they are relatively rare among late-summer flowering annuals.
Pollinators and Wildlife
The large, accessible blooms of Giants of California attract bees, butterflies, and hoverflies throughout the summer. The open-centred or semi-open petal arrangements allow pollinators to reach the nectar and pollen easily. A mass planting of Giants of California in August and September is consistently active with pollinator traffic throughout the warm parts of the day, providing a late-season nectar resource that complements the butterfly-attracting Verbena bonariensis and Tithonia at the same period.
Mass Planting for Maximum Impact
While Giants of California is excellent as a cut flower, it is also spectacular when planted in large groups in the border. A block of 12 or more plants at 45cm spacing creates a dramatic late-summer display that provides height, bold colour, and structure from July through to October. The tall, upright stems and large blooms give the planting a subtropical exuberance that is characteristic of the best late-summer border. Combine with Rudbeckia, Tithonia, Cleome, and annual grasses for a rich, warm-season display.
Powdery Mildew — The Main Risk
Powdery mildew is the most common and damaging problem for zinnias in the UK. The white coating appears on leaves in warm, humid conditions, particularly when overhead watering deposits moisture on the foliage. Prevention is straightforward: water at the base of the plant only, never overhead; space plants at 45cm to allow good air circulation; and avoid overhead irrigation systems in zinnia plantings. Once powdery mildew is established on leaves, remove affected material promptly and ensure air circulation is adequate. Healthy, well-spaced plants with base watering rarely suffer severely.
Open-Pollinated Heirloom — Seed Saving
As an open-pollinated heirloom variety, Giants of California can have its seed saved from the finest specimens at the end of the season. Allow a few selected blooms to remain on the plant until fully dry and the seed head has browned. Remove the dry flower heads, extract the arrow-shaped zinnia seeds, and dry thoroughly on kitchen paper. Store in a labelled paper envelope in a cool, dry place over winter. Home-saved Giants of California seed produces plants of the same character as the parent, though colour distribution may vary season to season — one of the pleasures of an open-pollinated variety.
Sowing & Flowering Calendar
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
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| Sow individually (Mar–May; 21–24°C; 5–7 days) |
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| Grow on and harden off (May–Jun; bright; 18–20°C) |
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| Plant out (June; warm soil; 45cm; stake if needed) |
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| Huge blooms (Jul–Oct; full colour range; cut hard) |
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Common Problems & Solutions
| Problem | Likely Cause | What to Do |
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| Powdery mildew on leaves — white coating | Overhead watering; humid conditions; poor air circulation | Water at the base only — never overhead. Space plants to allow air circulation. Powdery mildew is the most common zinnia problem; prevention through correct watering is far more effective than treatment. |
| Wilting after transplanting; slow establishment | Root disturbance during transplanting; cold soil | Sow in individual modules and plant without disturbing the root ball. Plant only in June when soil is genuinely warm — cold soil causes slow, sulky establishment that powdery mildew then exploits. |
| Poor germination | Temperature too low; seed too old | Maintain 21–24°C consistently — lower temperatures give unreliable germination. Use fresh seed each season. |
| Few flowers; mainly leaves | Over-rich soil; not deadheaded; not pinched | Pinch growing tip at 10–15cm. Deadhead consistently. Avoid very rich soil. Feed with high-potash (not high-nitrogen) fertiliser once flowering begins. |
Plant Specifications
Enormous 12–15cm double blooms in every colour from July to October — the 1919 heirloom that still sets the standard for large-flowered cutting zinnias
Sow individually in modules March–May at 21–24°C (no root disturbance — individual pots essential; 5–7 days). Pinch at 10–15cm for multi-stemmed plants. Plant out in June when soil is genuinely warm at 45cm spacing. Water at base only — never overhead. Feed fortnightly from flowering. Cut stems long to a leaf node for continuous production July–October.
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