Why Your Plants Look Sick in Autumn: Identifying & Fixing the Most Common Nutrient Deficiencies in NZ Gardens

Autumn is when plants start showing their stress. Cooler soils, slower root activity, and reduced nutrient uptake mean deficiencies that were hiding all summer suddenly become very visible.

The good news? Your plants are giving you clear signals. You just need to know what to look for — and act before winter locks the problem in.

🟡 Yellow Leaves

Yellowing is the most common autumn complaint. Where it appears tells you a lot. If it starts on older lower leaves and moves upward, you're likely dealing with magnesium or nitrogen. If young tip growth goes yellow while the veins stay green, iron deficiency is the more probable cause. Whole-plant yellowing across all growth at once usually points to a general nutrient lockout — common when cold soils reduce root function and uptake slows dramatically.

🟣 Purple or Reddish Leaves

Purple or reddish colouring in leaves that are normally green is a classic sign of phosphorus stress — but in autumn it can also indicate potassium deficiency affecting how the plant translocates nutrients internally. It often appears on the undersides of leaves first, then spreads to the upper surface. Stems can take on a reddish tinge too. If you're seeing this across dahlias, tomatoes, or brassicas, take it seriously — it compounds quickly in cooler conditions.

🌿 Weak or Spindly Stems

Stems that flop, snap easily, or look thin and watery are usually a calcium issue. Calcium is critical for cell wall strength, and it moves slowly through the plant — meaning by the time stems go weak, the deficiency has been building for a while. Poor soil structure and irregular watering both make calcium uptake worse, so autumn's variable rainfall doesn't help.

🐢 Slow or Stunted Growth

If your plants have just stopped — minimal new growth, small leaves, everything looking a bit stuck — potassium is the first place to look. Potassium drives the energy processes that keep plants actively growing and building reserves. A deficiency here heading into winter leaves plants poorly positioned for spring recovery.

🌀 Leaf Curl

Leaf curl can be environmental (wind, cold) but when it persists across multiple plants in sheltered positions, it usually signals calcium or boron deficiency. Young leaves that curl inward and look distorted rather than simply wilted are a good indicator. You'll often see it alongside browning leaf edges.

The Deficiencies Behind the Symptoms

🔶 Iron Deficiency

Iron deficiency shows up as interveinal chlorosis — leaves turn yellow but the veins remain green, creating a distinctive striped look. It's most common in high-pH soils where iron becomes chemically locked and unavailable to roots, even if it's present in the ground.

The fix is a chelated iron product applied as a foliar spray or soil drench, which bypasses pH-related lockout. Our EDTA Iron Chelate is formulated specifically for fast correction and is the most direct solution for autumn interveinal yellowing.

🟢 Magnesium Deficiency

Magnesium is the central atom in chlorophyll — so when it runs low, green fades fast. Deficiency typically starts on older leaves, with yellowing between the veins while the veins themselves stay green. It's extremely common in NZ autumn soils, particularly in high-rainfall areas where magnesium leaches readily.

Our EDTA Magnesium Chelate is available in chelated form for fast foliar uptake, or you can use Epsom salts as a budget option for mild cases. For gardens that have been hungry for a while, the chelate will give you faster results.

🔵 Calcium Issues

Calcium is responsible for strong cell walls, healthy root tips, and structural integrity throughout the plant. Deficiency causes tip burn, blossom end rot (in fruiting plants), distorted new growth, and those weak floppy stems mentioned above.

Calcium doesn't move well through the plant, so foliar application directly to affected growth is often more effective than soil application alone. Our Calcium Ammonium Nitrate delivers a combined calcium and nitrogen hit — useful heading into autumn when both are often short at once. For pH correction alongside calcium improvement, Ag Lime applied to the soil this autumn will improve availability going into spring.

🟣 Potassium Deficiency

Potassium is the nutrient that keeps everything running — water regulation, disease resistance, energy transfer, and root development. Deficiency shows up as scorched or browning leaf edges (often called scorch), purple tints, and plants that simply seem to stall.

It's also the nutrient most worth addressing heading into winter, because potassium builds the reserves plants draw on during dormancy. Our Sulphate of Potash is a clean, low-chloride option suitable for all garden plants, or if you want a combined potassium and phosphorus hit, Potash Sulphur SuperPlus covers both in one application.

Fix It Before Winter

Autumn is actually the ideal time to correct deficiencies — plants are still active enough to respond, and corrections made now carry into spring recovery. Waiting until growth stalls in winter means the problem is locked in until September.

If you're not sure where to start, browse our Raw Fertilisers and Nutrients range — everything is available in small quantities so you can target specific deficiencies without overcommitting.

Got a question about what you're seeing in your garden? Get in touch — we're always happy to help diagnose.