Plant Profile & Key Characteristics
Overview: Oxalis obtusa (obtusa wood-sorrel) is a small bulbous perennial geophyte 10-15 cm tall that goes dormant in summer. It is grown as an ornamental for its cheerful cup-shaped flowers over clover-like foliage.
Quick Specifications
| Specification | Attribute |
|---|---|
| Plant Type / Life Cycle | Perennial (geophyte, summer-dormant) |
| Foliage Type | Deciduous (summer-dormant) |
| Mature Size | 10-15 cm (4-6 in) |
| Growth Rate | Moderate to fast |
| Root System | Bulbous geophyte |
| Toxicity | Contains oxalic acid – edible only in small amounts; large quantities can be harmful (soluble oxalates) |
| Hardiness | USDA zones 9-11 |
Growth Form & Structure
- Height & Spread: 10-15 cm (4-6 in); grows well in pots (start pots at ~3-6 L / 0.8-1.6 gal).
- Growth Habit & Form: Low clump-forming geophyte from small bulbs; can spread and become invasive.
- Lifespan & Maturity: Perennial that returns and multiplies from its bulbs each season.
Anatomy & Physical Features
- Leaves & Foliage: Green obcordate (heart-shaped) leaflets in threes forming a clover-like radial leaf.
- Flower Characteristics: Pink, white, orange, purple or yellow five-petalled flowers with a yellow throat, about 2.5 cm, in clusters (full detail in the Bloom guide).
Primary Uses & Landscape Value
- Functional & Ornamental Roles: Ornamental container, rockery and groundcover geophyte.
Care Guide: Planting, Soil & Watering
Overview: Oxalis obtusa is a bulbous, summer-dormant geophyte that grows from small bulbs in rich, well-draining soil kept evenly moist during its active season. It appreciates regular moisture while in leaf, then rests through its dormancy.
Quick Care Requirements
| Requirement | Ideal Condition |
|---|---|
| Planting Season | Spring to summer in warm zones; autumn to winter where temperatures drop below freezing |
| Growth Rate | Moderate to Fast |
| Soil Type | Rich, well-draining soil that holds some moisture (peat / organic) |
| Water Need | Moderate to High – keep evenly moist during active growth |
Planting & Establishment
- Best Planting Season: Plant at the start of the growing season – spring to summer in mild climates, or autumn to winter where hard frosts occur; set bulbs 2–5 cm (1–2 in) deep.
- How to Plant:
- Hole Preparation: Dig the hole noticeably larger than the root ball – roughly two to three times its size – so the roots settle into loose soil; bulbs go 2–5 cm (1–2 in) deep.
- Soil Amendments: Mix organic matter, humus and dried leaves into the backfill for a rich, moisture-retentive but still free-draining bed, then mulch the surface to hold moisture.
- Planting Depth: Set the root ball at soil level and firm the soil gently – not too hard, but firm enough that the plant does not sink when watered.
- Growth Speed & Expectations: Growth is moderate to fast during the active season; the clump fills out and can be lifted and divided when dormant.
Soil, Watering & Drainage Care
- Soil Mix & Drainage Needs: Use a rich, well-draining mix that still retains some moisture; in containers use rich soil with organic matter and drainage holes, and the bulbs can be lifted and stored during dormancy. It grows in full sun to half shade.
- Watering Schedule & Technique:
- Young / Newly Planted: Water daily for about the first two weeks after planting (preferably in the morning) to settle the plant, keeping the soil consistently moist.
- Established: Keep the soil evenly moist through active growth, watering deeply but never leaving it waterlogged.
- Seasonal Adjustments: Reduce watering as the foliage dies back into dormancy and keep it on the dry side while resting; resume regular watering when new growth appears.
Climate, Light & Hardiness
Oxalis obtusa is a South African winter-growing geophyte suited to Mediterranean and subtropical climates, growing in full sun to half shade during its active cool season and dying back to the bulb in summer. It needs a genuinely mild-to-cool climate, being hardy only from USDA zone 9 through 11.
Quick Climate Specifications
| Climate Zones | Mediterranean, subtropical |
|---|---|
| USDA Hardiness Zones | 9–11 |
| Light Conditions | Full sun to half shade |
Sunlight & Light Exposure
Optimal Light Setup
Full sun to half shade during its autumn-to-spring growing season, when the plant is in leaf; it is dormant and leafless through summer regardless of light.
Temperature Extremes & Hardiness
USDA Hardiness & Regional Limits
Hardy from zone 9 through 11, reflecting a preference for mild winters and warm, dry summers typical of a Mediterranean-type climate; it is planted and grows through autumn, winter, and spring, going summer-dormant as temperatures rise.
Cold Damage & Winter Protection
The bulb needs a period of at least a month with temperatures under about 5°C (41°F) to properly break summer dormancy and trigger its autumn–winter growth cycle; where winters do not reach this, growth and flowering may be inconsistent.
Growing in Pots, Balconies & Indoors
Oxalis obtusa (Oxalis obtusa) is a compact, bulbous wood-sorrel that is tailor-made for pots – small, shallow-rooted and tidy, it suits a windowsill, a small container or the front of a balcony display. The grower does not list it as a houseplant; it wants strong light, so grow it on a bright, sunny windowsill rather than deep in a room.
Quick Details
- Container Size: About 3-6 L (0.8-1.6 gal) to start (the grower’s guide), potting up as it fills
- Indoor Suitability: Best on a bright, sunny windowsill rather than a dim room
- Best Spot: Full sun / half shade
- Soil & Drainage: Rich, free-draining mix with drainage holes; drainage matters for the bulbs
- Watering in Pots: Average to generous while in leaf; ease right off as it dies back into dormancy
Indoors & On the Windowsill
- On a sunny windowsill: The grower does not rate it as a houseplant, but its small size suits a bright, sunny sill where it gets the direct light it needs.
- Dormancy indoors: Oxalis naturally rests after flowering – foliage dies back and the pot looks bare. Keep it cool and nearly dry through dormancy, then resume watering to bring it back.
- Safety: Oxalis foliage contains oxalates and can upset pets or children if eaten in quantity – site pots out of reach and do not treat it as edible.
On a Balcony or Patio
- Light & shelter: Happy on a bright balcony or patio in a small pot; being low and light it is easily blown over, so group pots or use a heavier container in exposed spots.
- Watering: Small pots dry quickly in sun and wind – check often while the plant is in active leaf.
Choosing the Pot, Soil & Long-Term Care
- Pot & drainage: Start around 3-6 L with good drainage holes (the grower stresses drainage for the bulbs). Use a pot with strong drainage holes and skip a standing water saucer – constantly wet feet are the quickest route to root rot.
- Potting mix: A rich, free-draining mix with organic matter, as the grower advises.
- Potting up & soil refresh: Each time the plant fills its pot, move up a size until it reaches the size you want; because potting mix loses vigour over time, refresh part of the soil when you pot on.
- Feeding: Potted plants exhaust their food faster than those in the ground, as watering leaches nutrients out; feed lightly but more regularly through the growing season.
Grower’s Container Tip
- Pot up in stages: The grower starts in a small pot (about 3-6 L) and steps up to a larger one each time the plant fills the current one, until it reaches the desired size – this keeps the soil fresh and makes care efficient. In the final large pot, swap out part of the soil every few years; from the side of a big pot it is easy to replace some soil without disturbing the whole plant.
- Lift the bulbs for dormancy: In the dormant season you can lift the bulbs out of the soil, store them, and replant when growth restarts.
Fertilizing & Feeding
Oxalis obtusa is a small, summer-dormant bulb that needs only light, balanced feeding during its cool-season active growth to support its colorful flowers.
Quick Fertilizing Specs
| Feeding Level | Light – a small bulb that needs only modest feeding during its active season |
|---|---|
| young_feeding | Light, balanced feeding as newly planted bulbs establish roots and their first leaves |
| mature_feeding | Light, balanced feeding through the cool-season leaf growth and bloom period |
| Main Nutrient Priority | Balanced feeding is enough to support healthy leaves and the colorful flowers this plant is grown for |
| Best Feeding Period | Through active leaf growth in the cooler months, before the plant goes summer-dormant |
| Organic Options | A light topdressing of compost worked around the bulbs at the start of active growth |
| Micronutrients | No routine micronutrient program is typically needed for this small bulb |
| Soil pH | Prefers well-drained soil and tolerates a fairly wide pH range |
| Reduce/Stop Feeding | Feeding is unnecessary once the foliage yellows and the plant goes summer-dormant |
Feeding by Growth Stage
Establishment After Planting
Support root and early leaf development as newly planted bulbs get established.
| Nutrient direction | A light, balanced feeding worked into well-drained soil at planting supports steady early root and leaf growth. |
|---|---|
| Timing | At planting and through the first flush of leaves. |
| Frequency | Light feeding is enough while the bulbs are still establishing. |
| Fertilizer types | A balanced, complete fertilizer worked lightly into the soil at planting, Organic matter such as compost blended into the planting hole |
| Precautions | Avoid placing concentrated fertilizer in direct contact with the bulbs, which can cause damage. |
Active Growth and Bloom
Support the cool-season leaf growth and colorful bloom this small bulb is grown for.
| Nutrient direction | Continued light, balanced feeding through active growth supports healthy foliage and flowering. |
|---|---|
| Timing | Through the cool-season active growth and bloom period. |
| Frequency | Light, infrequent feeding through this stage is generally enough. |
| Fertilizer types | A light, balanced feeding applied during active leaf growth, A diluted liquid feed is a gentle option for container-grown bulbs |
| Precautions | Rich or heavy feeding is rarely needed and can favor leafy growth over flowering in this species. |
Summer Dormancy
Let the bulb rest through its natural summer-deciduous dormant period.
| Nutrient direction | None – withhold fertilizer once the foliage yellows and the plant enters summer dormancy. |
|---|---|
| Timing | Through summer dormancy, after the foliage dies back. |
| Frequency | No feeding at this stage. |
| Fertilizer types | No feeding at this stage – the dormant bulb has no active leaves to support |
| Precautions | Do not fertilize dormant bulbs; resume light feeding only once new leaf growth appears in the cooler months. |
Nutrient Deficiency Signs & Corrections
Leaves look pale green to yellowish with weak, sparse growth during the active season.
| Possible causes | Nitrogen deficiency, Very poor, depleted soil around the bulbs, Bulbs planted too deep or in poorly draining soil restricting root function |
|---|---|
| Appears first on | Overall leaf color during active growth. |
| What to check | Soil richness and drainage, Planting depth of the bulbs, Recent feeding history |
| Possible correction | A light, balanced feeding at the start of active growth is usually enough to restore healthy leaf color. |
New leaves look pale to yellowish while the veins remain visibly greener.
| Possible causes | Iron deficiency (chlorosis), High soil pH limiting iron availability, Waterlogged or poorly aerated soil |
|---|---|
| Appears first on | Newest leaves. |
| What to check | Soil pH, Drainage in the planting area or container, Whether yellowing is specific to new growth |
| Possible correction | Correct soil pH toward a more favorable range and apply a chelated iron product per its label if chlorosis is confirmed. |
Flowering is sparse or weak and older leaf margins show a scorched, dry look.
| Possible causes | Potassium deficiency, Bulbs that are undersized, crowded, or overdue for division, Inconsistent watering through the active growth period |
|---|---|
| Appears first on | Older leaf margins and overall flower production. |
| What to check | Bulb size and how long since the clump was last divided, Watering consistency during active growth, Recent feeding history |
| Possible correction | A balanced feeding with adequate potassium, along with dividing overcrowded bulbs, typically improves flowering. |
Pruning, Training & Seasonal Maintenance
Oxalis obtusa is a small, clover-leaved bulb (10–15 cm) from a summer-dormant, winter-growing South African cycle – it flowers from autumn through spring, then dies back and rests through summer. It has no stems or wood to train, so "pruning" here means light grooming: deadheading spent flower stalks during the growing season and clearing foliage only once it has yellowed on its own. Because next season’s flowers come from the bulb rather than old growth, grooming timing does not affect flowering, but removing green leaves early does reduce the reserves the bulb can store for next year.
Quick Pruning & Maintenance Specs
| Pruning Tolerance | Low – grooming only, no structural cuts |
| Primary Pruning Window | Autumn through spring, during active growth and bloom (ongoing deadheading) |
| Secondary Maintenance Window | Once foliage yellows heading into summer dormancy |
| Typical Frequency | Light, as-needed, every few weeks while in bloom |
| Main Objective | Tidy appearance and limiting unwanted self-seeding |
| Young Plant Priority | Leave undisturbed while the young bulb/offset establishes |
| Mature Plant Priority | Regular deadheading; remove foliage only after it yellows |
| Best Tool Type | Fingers or fine precision snips |
| Avoid Heavy Pruning During | While foliage is still green and actively growing |
Pruning by Plant Age & Size
Newly Established
Do not cut back a newly planted bulb or offset; let it root and put out leaves undisturbed through its first season.
Young / Formative
No structural training – Oxalis obtusa has no framework to shape. The only formative task is starting to deadhead once flowering begins.
Mature / Established
Deadhead through the bloom season and clear spent foliage as dormancy approaches. A clump that flowers poorly after several years is refreshed by dividing and replanting bulbs, not by cutting back top growth.
Seasonal Pruning Schedule
Growth & Bloom Season (Autumn–Spring)
Objective: tidiness and reduced self-sowing. Remove faded flower stalks at their base as each finishes. Do not remove green leaves. New flowers continue from the bulb, so deadheading does not reduce the display.
Pre-Dormancy (as leaves yellow, heading into summer)
Objective: clean transition into summer rest. Remove foliage only once it has yellowed or dried; do not pull green leaves or disturb the bulb itself. Premature removal of green foliage cuts into the bulb’s stored reserves for the following season.
Special Growth to Remove
- Spent flower stems – deadhead at the base of the stalk.
- Yellowed or dried foliage once dormancy begins.
- This species is noted as invasive in cultivation; deadheading before seed set helps limit unwanted spread in beds where that matters.
Container & Small-Space Pruning
Oxalis obtusa is not grown as an indoor houseplant, so its small-space role is as a pot or rockery plant. In containers, deadhead more often since spent blooms are more visible at this scale. Once foliage yellows for dormancy, stop watering and let the pot rest dry through summer rather than continuing to trim and water it as if still in growth.
Tools
- Precision snips or fine bypass snips for flower stalks and leaves.
- Fingers work well for pinching off spent blooms cleanly.
Tool Hygiene & Disease Risk
Routine – clean, sharp tools are sufficient. The powdery mildew and fungal issues noted for this species are foliar/airborne, not typically spread through grooming cuts.
Cut Placement & Technique
Deadhead by removing the flower stalk at its base near the leaf mound, not partway up the stem. Snip or gently pull dead leaves at their base once fully yellowed; avoid tearing into the crown.
Heavy Pruning & Rejuvenation
Not applicable – there is no woody framework to renew by hard pruning. A declining clump is rejuvenated by lifting and dividing bulbs during dormancy, not by cutting back top growth.
Post-Pruning Care
No extra feeding is needed after routine deadheading. Once foliage is cut back for dormancy, withhold water and keep the bulb dry through its summer rest.
Common Pruning Mistakes
- Cutting back green, still-photosynthesizing leaves before they yellow naturally.
- Continuing to water through the summer dormant period.
- Skipping deadheading in beds where volunteer seedlings from this invasive-prone species are unwanted.
When NOT to Prune
Avoid cutting back green, actively growing foliage. Do not disturb the plant during active bloom beyond simple deadheading, and do not dig into or prune around it during summer dormancy while the bulb is resting.
Recommended Tools (by task)
- Deadheading & foliage cleanup → precision_snips – clean removal of thin flower stalks and soft leaves without crushing the small clump.
Propagation Guide
Oxalis obtusa is best propagated by dividing and replanting its bulbs during summer dormancy, which the site itself says is easier and more dependable than starting from seed. The small, sometimes deeply buried bulbs must be dug out gently to avoid damaging them, which makes bulb propagation Moderate rather than simply easy.
Quick Propagation Specs
| Best Propagation Method | Bulb division (during dormancy) |
| Alternative Methods | Seed |
| Propagation Difficulty | Moderate – bulbs are small, can sit deep, and are easily lost or damaged when dug |
| Seed True-to-Type | Usually true-to-type (species-level, not a named hybrid) |
| Vegetative True-to-Type | Yes |
| Best Propagation Season | Spring above 5°C (41°F), or autumn where winter temperatures fall below 0°C (32°F) |
| Seed Germination Time | 3-5 weeks |
| Seed Sowing Depth | 0.5 cm (0.25 in) |
Growing from Seed
Seed Handling & Viability
Store seed in a dry, dark place, ideally refrigerated, until sowing.
Dormancy & Pre-Sowing Treatment
The site notes a chilling requirement of at least one month below 5°C (41°F); giving seed (and dormant bulbs) this cold period helps trigger normal growth for this winter-growing, summer-dormant species.
Sowing Season
Spring above 5°C (41°F), or autumn where winter temperatures fall below 0°C (32°F).
Sowing Depth & Orientation
Sow about 0.5 cm (0.25 in) deep, spaced roughly 2 x 3 cm (0.8-1.2 in) apart; the site notes seeds do need to be buried, not left on the surface.
Germination Conditions
Keep the soil moist with high humidity; do not let it dry out.
Germination Time
3-5 weeks.
Seedling Care
Grow seedlings on in moist soil with high humidity and half shade.
Rhizomes, Tubers, Bulbs & Corms
Bulb Division: best carried out in the dormant season by separating bulbs from the parent clump. The site flags the main practical difficulty: bulbs are small, can sit quite deep, and are easy to miss, so dig for them gently and avoid letting them dry out while lifting – if bulbs can’t be located, the whole plant can instead be dug up and divided by hand, though mishandling risks losing the plant. Replant divided bulbs about 2-5 cm (1-2 in) deep in moist, rich soil at the same spring/autumn timing above, and keep them watered in.
Bloom & Flower Guide
Quick Details
- Bloom Season: Autumn through spring – a cool-season bloomer
- Time to First Bloom: Each season, once the dormant bulbs wake and put up growth
- Flower Appearance: Cheerful five-petaled flowers to about 2.5 cm in pink, white, orange, purple or yellow, each with a yellow center, in small clusters
- Edibility: Grown as an ornamental for its flowers – not used as an edible flower here
Detailed Blooming Overview
Oxalis obtusa is a small bulb from a winter-rainfall climate, so it does its growing and flowering through the cooler months and rests in summer. The funnel-shaped flowers come in a lovely range of colors, each lit by a yellow eye, and like other wood-sorrels they open wide in sunshine and close up in dull weather or at night.
Flower Characteristics
- Five-petaled flowers to about 2.5 cm with a yellow center.
- Colors range through pink, white, orange, purple and yellow.
- Open in sun and close in dull weather or at night.
Bloom Care & Thinning
- Managing the bloom: Give it a bright, sunny spot to keep the flowers opening fully, and let the plant die back naturally into summer dormancy after flowering.
- Pollination: By bees and other small insects.
Pests, Diseases & Troubleshooting
Overview: Oxalis obtusa (a bulbous South African wood sorrel) is a bulb/tuber-forming wood sorrel whose signature disease is a bright orange rust, alongside powdery mildew and the sap-sucking insects – aphids, spider mites and whitefly – that trouble soft-leaved plants; underground, bulbs and tubers rot readily if kept wet in dormancy. Airy, evenly-drained culture and prompt removal of rusted leaves keep it healthy.
Quick Troubleshooting Overview
| Issue / Pest | Main Symptom | Primary Solution |
|---|---|---|
| Oxalis rust | Bright orange pustules on leaf undersides | Remove infected leaves, improve airflow |
| Powdery mildew | White powdery coating on leaves | Airflow, avoid wet foliage, sulfur/bicarb spray |
| Aphids | Sticky, curled leaves and stems; honeydew | Water jet, insecticidal soap |
| Spider mites | Fine stippling and webbing in dry heat | Raise humidity, rinse, neem/soap |
| Whitefly & fungus gnats | White flies / gnats around potted plants | Sticky traps, let soil dry, soap/neem |
| Bulb / root rot | Mushy bulbs; failure to sprout | Dry dormancy, gritty mix, discard soft bulbs |
Common Pests & Management
- Aphids (Aphididae):
- Symptoms: Soft green, grey or black insects clustered on tender leaves, stems and flower stalks, causing curling, sticky honeydew and sooty mould, and able to spread viruses.
- Prevention & Control: Rinse off with a firm water spray and encourage ladybirds and lacewings; control ants. Use insecticidal soap or neem oil on stubborn colonies, covering leaf undersides and growing tips.
- Spider mites (Tetranychus urticae):
- Symptoms: Fine pale stippling and a dusty, bronzed look with delicate webbing on leaf undersides in hot, dry, still air – a frequent problem on indoor Oxalis such as the purple shamrock.
- Prevention & Control: Raise humidity and rinse the foliage regularly, then treat with insecticidal soap or neem oil on the undersides, repeating to catch hatchlings; a spell of cooler, more humid air alone often checks them.
- Whitefly (Trialeurodes vaporariorum):
- Symptoms: Tiny white moth-like insects that rise in a cloud when disturbed, feeding on leaf undersides with honeydew and sooty mould, mostly indoors and under glass.
- Prevention & Control: Use yellow sticky traps to monitor and thin adults and spray insecticidal soap or neem oil on the undersides, repeating; good ventilation limits them.
- Fungus gnats (Bradysia spp.):
- Symptoms: Small dark flies drifting around potted plants; their larvae live in constantly moist compost and can nibble fine roots and tubers of seedlings and weak plants.
- Prevention & Control: Let the top of the compost dry between waterings, use yellow sticky traps for adults, and apply a Bacillus thuringiensis israelensis (Bti) or beneficial-nematode drench for larvae in heavy cases.
Diseases & Fungal Issues
- Oxalis rust (Puccinia oxalidis):
- Symptoms: Bright orange to rusty-brown powdery pustules on the undersides of the leaves, with yellow flecking above; heavy infection distorts and drops foliage. The most characteristic disease of the genus.
- Treatment & Management: Pick off and destroy infected leaves at the first pustules, avoid wetting the foliage, and improve air circulation and spacing. Clear fallen debris; where rust is persistent a sulphur fungicide gives protection, but sanitation and airflow are primary.
- Powdery mildew (Erysiphales):
- Symptoms: A white powdery coating on leaves and stems in warm days with humid nights and stagnant air, as flagged in the source, yellowing and weakening affected leaves.
- Treatment & Management: Improve airflow and light, avoid overhead watering and excess nitrogen, and remove affected leaves. A potassium-bicarbonate or sulphur spray helps in stubborn cases.
- Bulb, tuber and root rot (Pythium / Fusarium / Sclerotinia spp.):
- Symptoms: Soft, brown, sometimes foul bulbs and tubers that fail to sprout or rot in growth, plus stem-base collapse, after wet, poorly drained soil or watering through dormancy.
- Treatment & Management: Grow in gritty, free-draining soil, water only in active growth, and keep bulbs and tubers dry during their rest; lift and store dry in wet climates. Discard any soft, rotten storage organs rather than replanting.
- Leaf spot / downy patches (various fungi):
- Symptoms: Brown or purple leaf spots and, in damp weather, downy patches, mostly cosmetic on vigorous plants.
- Treatment & Management: Remove affected leaves, keep foliage dry, and improve airflow; rarely needs more than tidy culture.
Growing Difficulties & Abiotic Problems
- Bulb / tuber rot from overwatering:
- Cause & Symptoms: The main killer: bulbs and tubers left wet – especially during their natural dormancy – rot instead of resting.
- Correction Strategy: Reduce or stop watering as the foliage dies back, use free-draining soil and pots with drainage, and resume watering only when new growth appears.
- Invasiveness / self-seeding spread:
- Cause & Symptoms: Many Oxalis, notably Bermuda buttercup, spread aggressively by bulbils and seed and can become hard-to-eradicate weeds, as the source warns.
- Correction Strategy: Grow contained in pots or confined beds, deadhead before seed sets, and remove stray bulbils carefully; never compost the bulbils of weedy species.
- Dormancy die-back (mistaken for death):
- Cause & Symptoms: Many species naturally collapse and lose their leaves after flowering, which is often mistaken for disease.
- Correction Strategy: Recognise this as normal rest: keep the pot dry and cool through dormancy and growth will return; do not overwater trying to ‘revive’ it.
- Leaf scorch and faded colour:
- Cause & Symptoms: In fierce midday sun tender or coloured forms (e.g. purple shamrock) can scorch, while too little light fades the colour and stretches the plant.
- Correction Strategy: Give bright light but shelter from harsh midday sun for coloured indoor types; for green species, more sun deepens vigour without scorch.
What Helps – Products & Tools
Keep to gentle tools: a water jet, insecticidal soap and neem oil for aphids, mites, whitefly and fungus gnats, yellow sticky traps and a Bti drench for gnats, and a sulphur or potassium-bicarbonate fungicide for the rust and powdery mildew that define this genus. The best preventives are gritty free-draining soil, airflow, dry dormancy and prompt removal of rusted leaves. Note the leaves contain oxalic acid, so keep them from pets and grazing.
