The method
How the planner works out your dates
Growmanac reads about a decade of daily low temperatures for your exact location, takes each year's coldest day and averages them — that figure is your hardiness zone. It then checks which crops have a sowing window open this month in your hemisphere. This page explains that method, and the three things the plan honestly cannot see for you. The planner itself is on the home page, and repeated below.
How it works
Two things decide what grows well for you: how cold your winters get and what month it is. Growmanac looks up around a decade of your location’s daily low temperatures, takes each year’s coldest day, and averages them — that average low is what gardeners call your hardiness zone. A lower zone means hardier, cool-season crops; a higher zone opens up tender, warm-season ones outdoors.
Then it checks the calendar. Every crop has a sowing window; we show the ones whose window is open now, in your hemisphere (the Southern hemisphere’s seasons are flipped, and so is the plan). On a balcony or windowsill, tender crops still work because the space is sheltered — so the list adapts to where you’ll actually grow.
What the plan can’t see
Being straight about the limits is more useful than pretending there aren’t any. Three things shape your result that the planner has no way of knowing:
- Hours of direct sun on your actual spot. This is the big one. A north-facing balcony two floors down from a taller building can get a couple of hours in high summer, and no list of crops fixes that. Watch the spot for a morning and count.
- Your soil or compost. Drainage and depth decide whether roots go anywhere. A carrot needs about 25–30 cm of loose depth; a lettuce is happy in 15 cm.
- This particular year. The plan is built from long-run climate normals, not this week’s forecast. A late frost or a cold spring can shift things by two or three weeks, so check the forecast before you sow something tender.
Why the same crop has a different window somewhere else
Two numbers move a sowing window: when your frosts stop and start, and how much daylight you get. Frost sets the outer walls — tender crops can’t go out before the last one, and have to finish before the first. Daylight sets the pace inside those walls; the same tomato will ripen faster at a lower latitude with longer, stronger light.
That’s also why "sow in March" advice from a gardening book is a coin flip. March is a different month in Lisbon, Leeds and Lima. The planner works from your location instead of a national average, which is the whole reason it exists.
Balcony, windowsill, or bed — what actually changes
A container warms up faster than open ground in spring, which buys you an earlier start. It also dries out much faster in summer — a pot in full sun may need water daily in a heatwave where a bed can go days. That trade is the main difference between growing on a balcony and in a garden.
A windowsill is limited by light, not warmth, which is why the plan leans towards herbs, salad leaves and microgreens there: crops you harvest as leaves, not as fruit. A bed lifts the depth and water limits, so the list opens up — but it exposes you to frost the sheltered balcony was protecting you from.
If you think you’re late
You’re probably not, and it matters less than it feels. Every month has something open: fast crops like radish turn round in about four weeks, salad leaves can be cut repeatedly, and a slow crop started indoors now is ready to go out when its window opens. The planner deliberately shows what is open this month rather than what you missed — start with one thing that fits today.
This is guidance, not a guarantee — your microclimate, soil, and light matter too. Start small, and treat the first season as an experiment.
Common questions
Where does the climate data come from?
From Open-Meteo, a free climate service. Your browser asks it for roughly a decade of daily minimum temperatures at your coordinates, and the calculation happens on your device — nothing you type is sent to us and no account is involved. These are historical records rather than a forecast, which is what makes them useful for planning a season and useless for planning a weekend.
Why is my hardiness zone not enough on its own?
A zone describes one thing well: how cold your winter gets. It says nothing about how long your summer runs, and that is what decides whether a crop has time to finish. Two places can share a zone and still have seasons weeks apart. So the zone is used for cold-hardiness, and the calendar — your own frost dates and the month it is now — does the rest.
Can I grow food on a balcony or indoors?
Absolutely. Pick your space and the plan adapts — container-friendly crops for a balcony, herbs and microgreens for a windowsill, or a fuller list for a garden bed.
How much sun does a spot need before it is worth planting?
As a rule of thumb, leafy crops such as lettuce, spinach and most herbs want 3–4 hours of direct sun; root crops like radish and carrot want 4–6 hours; fruiting crops such as tomato, pepper and chilli want 6 hours or more. Sit outside on a free morning and note when the sun actually reaches the spot; it is usually less than people guess, and it is the one thing the planner cannot see for you.
How accurate are these dates?
They are built from long-run climate normals, which makes them a good guide to a typical year and a poor guide to this particular one. A late frost or a cold spring can move real dates by two or three weeks in either direction. Treat the plan as the shape of your season, then check the forecast before you sow anything frost-tender — and remember it cannot see your sun, your soil, or this year.
Do I need to buy anything before I start?
Very little. A container with drainage holes, potting compost, seed and water will grow most of what the planner suggests for a balcony or sill. The plan lists what each crop actually needs, so you can see the difference between what is required and what is merely nice to have before you spend anything.