The main trick to being successful with this plant is to choose early varieties. Pick a larger, long-day variety and you’ll still be waiting for fruit to ripen when the snow is on the ground.
When to Plant:
Start your seed indoors in the middle of April. Do not sow after the first of May as you won’t have time to ripen a crop.
Where to Plant
Full sun in rich, well drained soils
How to Plant:
Each seed gets its own pot because you don’t want to disturb the roots when you transplant. Use two to three seeds to each 10 cm pot because you’ll thin out the weakest seedlings once the plant gets to the three-leaf stage. (Three true leaves, not counting the first tiny seedling leaves)
Soil temperature is critical for success with this plant. Ensure the soil is at least 22-23C. Only use lukewarm water on the seedlings – never cold water. If you lower the soil temperature with cold water, the seeds will not be happy and will not germinate.
Once the seeds are up, grow them at an air temperature of 22-23C with good air circulation and high light levels. Feed with a regular, balanced house plant food (10-10-10 or half-strength fish emulsion) to keep the seedling growing.
This plant does not like to have its roots disturbed so transplant carefully.
Care & Maintenance
Harden melons off before you put them outdoors. If you move them directly from high heat to outside temperatures, you’ll stunt them and set them back.
Put the melons outside after all danger of frost and when the soil has warmed up. In my zone 4-5 garden, this is normally the middle of June. You can stretch black plastic mulch (garbage bags work well) over the planting area to warm it up as soon as the ground is workable. Then when the air temperatures are high enough, the soil under the black poly will be quite warm enough for melons. Weight the plastic edges so they don’t fly away in the wind. Some gardeners leave the black plastic in place for this crop and merely make a slit in the plastic to plant the potted watermelon in the ground under the poly.
Once they are large, how do you know they’re ripe? Look for when the rind on the bottom of the watermelon turns from white to pale yellow. Pale yellow means this baby is done.
Figure you’ll get (realistically) three melons from each vine.
Once planted, remember that watermelons are mostly water and you do have to provide them with enough to both grow the vine and fruit. They are also very shallow rooted plants and weeding next to the vine has to be done carefully or you’ll chop up the roots. So, don’t move the vines – they resent this big time.
Feed compost tea and/or fish emulsion every few weeks to keep them healthy and growing.