When to Plant Beans

A cold failure and a hot failure look nothing alike from the plant's side, and that is why a single date cannot describe this crop. Cold kills the seed before the plant exists: the seed takes up water, cannot start growing, and is colonised by soil organisms instead. Hot weather kills the crop after the plant is already growing well, because the failure is in pollination rather than in growth.

Iowa State's diagnostic note explains the hot one: above about 90°F during the day, pollen viability falls and pollination fails, so an unpollinated flower dies and drops. The University of Delaware's heat-stress note describes the same event from the pod side, reporting that high temperatures during flowering interfere with pollen release and reduce pollen quality, leaving short, twisted, lumpy pods with few seeds. The plant is not damaged. Its flowers simply never set.

The two failures also arrive on different schedules. The cold end is a condition of the bed on one morning; you can measure it, and it either passes or it does not. The hot end is a property of a fortnight, which is why a heat wave during flowering costs a whole flush of pods and a single hot afternoon costs nothing.

The three published heat thresholds are not a rounding difference. Maryland Extension puts the limit at 85°F, above which bean plants stop flowering or the flowers die. Iowa State names 90°F as the point where pollen viability falls. Minnesota Extension's vegetable team, writing during the 2026 heat, reports that flowers abort once temperatures stay above 95°F.

The gap is explained by what each figure is measuring rather than by one source being wrong. Different Phaseolus types carry different heat tolerance: the academic work on pollen under high temperature finds large differences between genotypes, and some cultivars hold viable pollen where others fail. A field trial that ran its heat for two days is also not the same stress as a garden that sat above 90°F for a fortnight, and a dry heat is not the same as a humid one.

So the honest reading is a range with a moving edge, and the practical version is simpler than the argument: treat the mid-80s as the point where you should be watching, and the mid-90s as the point where you should expect nothing to set until it breaks. Which of the three numbers applies to your garden depends on your cultivar and your weather, and neither of those is on a seed packet.

If the hot end is what limits the crop, the sowing date is really a decision about when the plants will flower. Bush beans reach flowering roughly six to eight weeks after sowing, so a bed sown in late May in a mid-latitude garden flowers in early-to-mid July, which in much of the country is the hottest fortnight of the year. That is the single most common way a bean crop is lost, and it is a scheduling mistake rather than a gardening one.

Succession sowing is the fix, and Maryland Extension gives the interval as every 2 to 3 weeks from the frost-free date through mid-to-late July. The point of the interval is not only a longer harvest. It is that a batch which flowers into a heat wave is followed a fortnight later by one that may not, so the risk is spread across the season instead of concentrated in whichever fortnight the single sowing happened to land in.

The other lever is the habit. Where summers run long and hot, the indeterminate climbing types are the safer bet, because Minnesota Extension notes they fail to set only a few pods in extreme heat and then continue producing once conditions return to normal. The crop pauses rather than ending, which is a different outcome from a bush type that set its whole crop into the same fortnight.

The bed matters most at the cold end, because that is where the seed either comes up or does not. Heavy or compacted ground works against beans for a mechanical reason: the two seed halves have to be pushed up through the surface, and crusted soil holds them down, which is why a bed that has been loosened and worked emerges more evenly than one that has not. Sow about an inch deep and keep the ground moist rather than soaked until the seedlings are up, since the same wet conditions that rot seed in spring will rot it in a wet spell later.

At the hot end, water is the one variable you control. Iowa State's guide to beans in the home garden asks for about an inch of water a week, especially through blossoming and pod development, and notes that hot, dry conditions during blossoming can drop many of the flowers before they set pods at all. Iowa State's blossom-drop note makes the same point from the failure side: green beans are particularly susceptible to dropping flowers under water stress.

That is worth planning around, because heat and drought usually arrive together. A bed that is mulched and watered through flowering turns a heat wave into a delay; a bed that dries out during the same fortnight turns it into an empty row. Neither the mulch nor the water changes the threshold, and neither has to: they change how much of the crop is still there when the heat breaks.