Manawatū-Whanganui's frost season runs long by North Island standards: Palmerston North averages nearly ten ground-frost days in July, closer to Waikato's figures than Wellington's, and the season doesn't properly clear until November. That makes it worth treating this region's tender crops on a similar timeline to Waikato rather than the milder parts of the lower North Island nearby.
Lettuce, spinach, silverbeet, peas, beetroot, carrots and dwarf beans are safe from September regardless. Tomatoes, capsicums, courgette and sweetcorn are worth waiting on until October or November, particularly on the open, flat sections common across the region, where cold air has nowhere to drain away on a clear night.
Those same open, flat sections bring a second problem that has nothing to do with frost: wind. A hard spring gale across the plains does more damage to a young seedling than a light frost ever would, snapping stems or simply drying plants out faster than their roots can keep up. A temporary windbreak, even something as simple as shade cloth on stakes around a new bed for the first few weeks, protects new plantings more effectively than mulch or feeding does at this stage. Once plants are established with a real root system, the wind risk drops off considerably.
Those same open, flat sections bring a second problem that has nothing to do with frost: wind.