Every seed packet and plant guide gives a days-to-harvest figure, and it's a genuinely useful starting point, radishes at 25-35 days, carrots at 60-80 days, tomatoes at 70-85 days from transplanting. But it's a guide, not a guarantee. Weather, soil temperature, how much sun a bed actually gets and how consistently it's been watered all shift that window earlier or later, sometimes by weeks either side of the figure on the packet. Treat the number as "start checking around now" rather than "pick on this exact day."
Treat the number as "start checking around now" rather than "pick on this exact day."
Root crops are the trickiest to judge from above ground, since the part you're waiting on is out of sight. Rather than pulling the whole plant to check, gently push soil back from the top of the root with your fingers to see how it's sizing up, then cover it back over if it needs more time. This works well for carrots, beetroot and radishes, and avoids wasting a plant that turns out to still be small.
Leafy greens split into two harvesting styles depending on the crop and how you're growing it. Cut-and-come-again crops like lettuce, silverbeet and spinach can be picked leaf by leaf from the outside, taking only what you need and letting the centre keep producing for weeks. Alternatively, a whole lettuce head can be cut once it's filled out, which suits varieties grown for a single dense head rather than loose leaves. Either way, picking outer leaves regularly rather than stripping a plant bare in one go keeps it producing longer.
Fruiting crops give the clearest visual and physical cues of any group. Colour change is the obvious one: capsicums deepen from green toward red or yellow the longer they're left, and a ripening tomato shifts from green to a full, even colour rather than staying blotchy. Tomatoes also give a physical cue worth learning: a ripe fruit gives slightly to a gentle squeeze rather than feeling rock hard, without being so soft it's starting to spoil. Zucchini is the opposite lesson: it's better picked young and often, since a fruit left even a few days too long swells rapidly into an oversized, seedy marrow.
If in doubt, it's almost always safer to check a little early than to leave a crop too long. Root vegetables left too long in the ground turn woody or split, zucchini turns to marrow within days of being ready, and lettuce past its best bolts to seed and turns bitter. A few plants harvested slightly ahead of their absolute peak beat a bed of overgrown, tough produce every time.