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Plant Basics

You don't need a biology degree to grow great vegetables — but knowing a handful of fundamentals helps you spot problems earlier, understand why advice works, and make better decisions when something doesn't go to plan. This article covers the things that happen inside and around a plant from seed to harvest.

Anatomy of a seedling

Every seed contains a miniature plant and a packed lunch. Knowing the parts helps you spot problems early.

  • Seed coat — the hard outer shell that protects the embryo. Soaking large seeds (beans, squash) for 12 hours softens the coat and speeds germination.
  • Cotyledons (seed leaves) — the first 'leaves' that emerge. They're not real leaves — they're food storage organs packed with starch and oils that fuel the seedling until it can photosynthesize. In most garden crops (dicots) there are exactly two; they're usually round, smooth and look nothing like the plant's actual leaves.
  • Hypocotyl — the stem section below the cotyledons. In tomatoes and sunflowers it stretches tall to push the cotyledons above the soil. If it's very long and thin, the seedling is leggy (not enough light).
  • True leaves — the first leaves that look like the adult plant. They appear above the cotyledons and mark the point where the seedling starts feeding itself through photosynthesis. This is the signal to prick out or pot up.
  • Primary root (radicle) — the very first thing to emerge from the seed, anchoring the plant and hunting for water. Within days it branches into a root system.

    • Cotyledons vs. true leaves

    This distinction matters because many care decisions (pricking out, feeding, diagnosing problems) depend on it.

  • Cotyledons are always the bottom-most pair. They emerge first, attached directly to the seed. In most vegetables they're oval or round with smooth edges.
  • True leaves grow from the centre, above the cotyledons. They have the shape, veining and texture of the adult plant — serrated edges on tomatoes, lobed on squash, narrow on onions.
  • Cotyledons eventually yellow and drop off — that's normal. The plant has used up the stored food. Don't remove them early; let them fall naturally.
  • Monocots (onion, leek, corn, grass) have only one cotyledon — it looks like a single thin blade. Dicots (tomato, pepper, bean, lettuce) have two.

    • Growth stages from seed to harvest

    Every plant goes through the same basic phases. Each one has different needs.

    Germination (Day 0–7)

    The seed absorbs water, the coat cracks, and the radicle pushes down. Need: warmth (20–25 °C), moisture, darkness or light depending on species. Most vegetable seeds don't need light to germinate — but check the packet for light-germinators like lettuce and celery.

    Cotyledon stage (Day 3–14)

    The cotyledons unfold and turn green. The seedling lives off its stored reserves and begins photosynthesizing. Need: bright light (12–16 h), moderate warmth (18–22 °C), gentle watering from below.

    Vegetative growth (Week 2–8)

    True leaves appear, the stem thickens, roots expand. The plant builds its 'solar panel' — more leaf surface means more energy. Need: light, water, nutrients (N-P-K), and space.

    Flowering & fruiting

    Triggered by day length, temperature or plant maturity. The plant shifts energy from making leaves to making flowers and fruit. Need: phosphorus and potassium, pollinators (for fruiting crops), consistent watering.

    Seed & senescence

    The plant puts its remaining energy into ripening seeds. Leaves yellow, growth stops. In annuals, this is the end of life. Harvest seeds at this stage if you want to save them for next year.

    How photosynthesis works (the short version)

    Photosynthesis is why light matters so much — it's how plants make food.

  • Leaves absorb light energy (mostly red and blue wavelengths) through chlorophyll — the green pigment.
  • That energy splits water molecules (H₂O) and combines the hydrogen with CO₂ from the air to build sugars (glucose).
  • Oxygen is released as a byproduct — that's why plants 'produce oxygen'.
  • The sugars fuel all growth: new leaves, roots, flowers, fruit. No light = no sugar = no growth. That's the entire reason a leggy seedling on a dark windowsill is weak.
  • This is also why leaf damage (pests, disease, hail) slows growth — fewer leaves means less photosynthesis, means less energy for everything.

    • Roots — more than anchors

    The hidden half of the plant does far more than hold it in place.

    Taproot (Pfahlwurzel)

    One thick main root going straight down: carrot, parsnip, beetroot, radish. These plants hate transplanting — the root forks or breaks when disturbed. Always direct-sow.

    Fibrous roots (Flachwurzler)

    A dense, shallow net of fine roots: lettuce, onion, strawberry. Transplant well, but dry out quickly because they're near the surface. Mulching helps enormously.

    Adventitious roots

    Roots that form on stems: tomatoes grow extra roots along buried stem sections (why deep planting works), basil cuttings root in a glass of water. Many propagation techniques exploit this.

  • Root hairs — invisible to the naked eye — are where most water and nutrient absorption happens. They're extremely fragile; rough transplanting shears them off, which is why freshly moved plants wilt even in wet soil.
  • Mycorrhiza — beneficial fungi that extend the root system by trading sugars for minerals. Present in healthy, undisturbed soil. Killed by heavy tilling and synthetic fungicides.

    • Nutrients: N-P-K and why it matters

    The three numbers on every fertiliser bag (e.g. 10-5-8) stand for nitrogen, phosphorus and potassium — the big three that plants consume in the largest quantities.

    Nitrogen (N) — the leaf builder

    Drives leaf and stem growth. Too little: pale yellow leaves, stunted growth. Too much: lush dark-green foliage but few flowers or fruit, and the soft growth attracts aphids. Leafy crops (lettuce, spinach, cabbage) are heavy nitrogen feeders.

    Phosphorus (P) — roots and flowers

    Essential for root development, flowering and fruit set. Too little: purple-tinged leaves, poor flowering, weak root system. Important at planting time and when flowers form. Bone meal is a classic organic phosphorus source.

    Potassium (K) — fruit and resilience

    Strengthens cell walls, improves drought and frost tolerance, promotes fruit ripening and flavour. Too little: brown leaf edges ('scorch'), poor fruit quality. Tomato feed is high in K for exactly this reason. Wood ash and comfrey tea are organic sources.

  • Secondary nutrients (calcium, magnesium, sulphur) and trace elements (iron, manganese, zinc, boron, etc.) are needed in smaller amounts. A diverse compost usually supplies them all. Deficiencies show as specific leaf patterns — interveinal yellowing (Mg), blossom end rot (Ca), distorted new growth (B).
  • Organic matter (compost, mulch) feeds soil life which releases nutrients slowly — a natural slow-release mechanism. Synthetic fertilisers work faster but bypass soil biology and wash out more easily.

    • Annual, biennial, perennial

    How long a plant lives determines when it flowers, sets seed, and dies — and how you manage it in the garden.

    Annual (einjährig)

    Completes its entire life cycle — seed to seed — in one season. Most vegetables: tomato, pepper, bean, lettuce, radish, courgette, basil. Plant in spring, harvest in summer/autumn, compost the spent plant.

    Biennial (zweijährig)

    Grows leaves in year one, flowers and sets seed in year two, then dies. Carrot, parsnip, beetroot, onion, leek, parsley, kale. We harvest them in year one before they bolt. If you want seeds, leave one plant to overwinter.

    Perennial (mehrjährig)

    Lives for years, flowering repeatedly. Rhubarb, asparagus, most herbs (rosemary, thyme, sage, mint), strawberry, berry bushes. Lower maintenance once established but need a permanent spot. Many herbs become woody after 3–4 years and benefit from replacement via cuttings.

    Plant families — why they matter

    Plants in the same botanical family share pest profiles, nutrient demands and diseases. That's the reason behind crop rotation: the same family in the same spot every year builds up family-specific soil pathogens. Knowing the families also helps you predict companion planting behaviour.

    Nightshades (Solanaceae)

    Tomato, pepper, chilli, aubergine, potato. All heavy feeders, all prone to blight and share the same soil diseases. Never follow a nightshade with another nightshade in the same bed.

    Cucurbits (Cucurbitaceae)

    Cucumber, courgette, squash, pumpkin, melon. Heavy feeders that love warmth and compost-rich soil. Prone to powdery mildew. Cross-pollinate within species — important if saving seeds.

    Brassicas (Brassicaceae)

    Cabbage, broccoli, kale, kohlrabi, radish, rocket, turnip. Attract the same pests (cabbage white butterfly, flea beetle) and suffer from clubroot if the soil is infected. Need lime and like firm soil.

    Legumes (Fabaceae)

    Bean, pea, clover, lupin. Fix atmospheric nitrogen into the soil through root nodules with Rhizobium bacteria. Great predecessor crop: the leftover nitrogen feeds the next planting. Don't fertilise legumes with extra nitrogen — it suppresses nodule formation.

    Alliums (Amaryllidaceae)

    Onion, garlic, leek, chives, shallot. Light feeders, repel many pests with their sulphur compounds. Classic companion for carrots (confuses carrot fly). Sensitive to waterlogging.

    Umbellifers (Apiaceae)

    Carrot, parsnip, celery, dill, coriander, fennel, parsley. Deep taproots that break up compacted soil. Attract beneficial hoverflies and parasitic wasps when allowed to flower. Most dislike transplanting.

    Common seedling problems

    Leggy / etiolated

    Tall, thin stems with wide gaps between leaves. Cause: too little light, too warm. Fix: add a grow light, lower the temperature, or move closer to the window.

    Damping off

    Seedlings keel over at soil level, the stem looks pinched and watery. Cause: fungal disease triggered by wet, stagnant conditions. Fix: improve ventilation, water from below, use sterile sowing mix, don't overcrowd.

    Yellow cotyledons (early)

    If cotyledons yellow before true leaves are well established, the seedling is running out of stored food faster than it can photosynthesize. Cause: too little light or germination took too long. Fix: more light, gentle half-strength feed.

    Curling or crispy leaf edges

    Cause: dry air (common above radiators), underwatering, or wind burn during hardening off. Fix: mist gently, water more consistently, slow down the hardening schedule.

    Purple-tinged leaves

    Cause: phosphorus deficiency, often triggered by cold soil or root damage. Common in tomatoes after transplanting into cold ground. Usually resolves as the soil warms — no action needed if the plant is growing.

    Understanding why things work makes you a better gardener than memorising what to do. If you know that tomatoes grow roots along buried stems, you'll bury them deep without needing to look it up. If you know legumes fix nitrogen, you'll naturally put them before heavy feeders in your rotation. The science is the shortcut.