When to plant tulips in Palmerston North: by 22 May

Tulip in Palmerston North: depth, winter cold, pests and pets

Plant tulips at a depth of two to three times the bulb’s height, at least twice the bulb’s width apart, pointed end up (RHS).

In Palmerston North the average daily temperature stays at 10 °C or lower for about 6 weeks a year (our estimate from ten years of monthly weather). For this bulb Wisconsin Horticulture Extension gives a cold period of 15–17 weeks at about 2–10 °C. That is less than the bulbs need, and without more cold they may not flower properly. Where winters are not cold enough for long enough, LSU AgCenter keeps tulip and hyacinth bulbs in the refrigerator for at least six to eight weeks before planting, and plants them only once the soil has cooled: warm soil can cancel the chilling. Keep fruit out of that refrigerator — ripening fruit gives off ethylene, which harms the flowers (Wisconsin Horticulture Extension).

Border tulips often fail to flower in their second year if left in the ground, so many gardeners grow them for one spring and plant fresh bulbs each autumn; Darwin hybrid and Kaufmanniana tulips reflower more reliably (RHS). Soils that stay wet in summer make reflowering difficult (RHS): in Palmerston North the three warmest months bring about 284 mm of rain together.

Grey squirrels often dig up tulip bulbs: lay chicken wire over the bed until the leaves appear, or plant daffodils among them, which squirrels avoid (RHS). Tulips are toxic to dogs, cats and horses, the bulb most of all (ASPCA).

Sources: RHS, Wisconsin Horticulture Extension, LSU AgCenter, ASPCA.