Seasonal Planning: What to Plant When

Why timing matters

The right week, the right seed. Two weeks early, it rots. Two weeks late, it bolts.

Timing beats variety choice. It beats spacing. It beats most amendments.

Yet most growers guess.

The three controls

Frost dates. Last spring frost sets the floor for tomatoes, peppers, squash. First autumn frost closes the warm-season window.

Soil temperature. Peas germinate at 5°C. Beans want 12°C. Corn wants 15°C. Below those numbers, you get rot and gaps.

Day length. Lengthening days flip spinach, lettuce, and onions into flowering mode. Once they bolt, leaves stop.

Every location runs a different calendar. Southern England and northern Scotland share crops and miss each other by months. There is no universal calendar. Only the principles.

Build the calendar

Start with the last spring frost date. Local meteorological records, allotment groups, or five years of your own notes will give it to you.

Cool-season crops go in before that date. Peas, broad beans, lettuce, spinach, radish, onion sets, brassica transplants: four to eight weeks ahead. Plant them late and peas stop podding, lettuce turns bitter, spinach bolts.

Warm-season crops go in after. Tomatoes, peppers, aubergines, cucumbers, beans, squash, corn: start indoors four to eight weeks before, transplant once soil hits their minimum. A tomato dropped into 8°C ground sits dormant. The same plant set out two weeks later catches it within days.

Work backward from the first autumn frost for late crops. Days to maturity on the packet, plus two weeks for shortening days. That gives you the latest sowing date. Succession planting at fortnightly intervals turns one glut into a steady supply.

Cool-season versus warm-season

Cool-season. Peas and broad beans take hard frost once established. Brassicas (cabbage, broccoli, kale, cauliflower, Brussels sprouts) handle moderate frost and sweeten with it. Lettuce, spinach, chard prefer 10 to 20°C and bolt above 25°C. Roots (carrots, parsnips, beetroot, turnips) sow cool, harvest across a wide window. Alliums go in autumn or early spring by type.

Warm-season. Tomatoes, peppers, aubergines are tropical perennials we grow as annuals. They want sustained warmth and long days. Cucurbits (cucumbers, squash, pumpkins, melons) sprint in heat and die at the first frost. Beans and corn anchor summer beds and slot into companion planting like the three sisters. Sweet potatoes need the longest hot season of the lot.

The transition weeks are the trickiest. Set warm-season transplants into beds where cool-season crops are finishing. Sow autumn brassicas into the gaps left by spent peas. Mind crop rotation: don't follow a crop with its cousin.

Stretch the season

Cheap covers buy weeks at both ends.

Cloches. Glass or plastic over single plants. Lift temperatures 3 to 5°C, block wind and pounding rain. A 2 L plastic bottle with the base cut off works. Skip the Victorian bell jars.

Row cover. Garden fleece over hoops or laid loose. Passes light and water. Adds 2 to 4°C, deflects light frost, and excludes flying pests. The single most useful piece of season-extension kit. Use it to warm spring soil, shield new transplants from late frost, and push autumn crops into early winter.

Cold frames. Bottomless boxes with a clear lid, set against a south-facing wall. Start cool-season crops in late winter. Hold salads past the first hard frosts. A frame on the sunny side of a house keeps lettuce going through all but the worst winters. Layer mulch inside to bank soil heat.

Climate is moving

Thirty-year averages are slipping. Frost dates shift. Heat waves stack up. Rain comes wrong.

Keep your own records. Five years of your own frost dates, soil temperatures, and outcomes beat any regional table built on old data. Mark which varieties carried a hot or dry year. Those are your future stock.

Seed saving from your best plants slowly tunes the line to your patch. Spread risk: more species, more varieties, more succession sowings. Lean on perennials that ride out the wobble on deep roots. And keep watching. Observation outperforms any fixed calendar when the climate stops repeating itself.