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I live in zone 3/4, I planted some Sugar Baby watermelons this year (first time!). I started them in my greenhouse and moved them outside around the time of the last frost since it was really warm out (+30C during the day, +13/14 during the night).

However, they haven't grown at all since I moved them - it's been a few weeks. The zucchini and squash (moved outside at the same time) are growing nicely in the same conditions.

I have them in full sun locations, they're growing in composted manure.

So, what's up? Are they too cold? They're not dying, but they're also not doing a whole lot. Should I just be patient?

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Yes, be patient. For one thing, there isn't much alternative. You could give them a little fish emulsion or compost tea, but I doubt I'd bother. If you have a wall of water container, I might try that just to warm the watermelon up a bit. If you do, fill it up well so it doesn't fall over onto the plant as the water gradually evaporates.

Cucurbits are very intolerant of root disturbance. That may be the cause, and it may not. Purely speculative, but perhaps the zucchini roots weren't so near the edge of the pot at the time of transplanting so got disturbed less, perhaps because the plants were smaller or the pots bigger. Or that's just the way the soil ball crumbled during transplanting. Maybe not too. For next year, if it's helpful, you might try planting direct instead, or also. One year I started cucumbers indoors about 3-4 weeks ahead, then planted a couple seeds nearby on the same day I transplanted the indoor ones. A month later you couldn't tell which was which.

Generally the first thing a transplanted plant does is ensure its roots are well settled and able to grow. The watermelon may be plenty busy underground even though no change is visible at the surface. As you know, you're asking the watermelon to thrive well outside its optimal temperature range. It's entirely possible to grow some fine watermelons. But they aren't going to be as cooperative with you as are plants more suited to your zone. It sounds like you've given them an optimal home in terms of soil and sun. Patient, fingers crossed, mouth watering already...

  • Thanks for your response! One of the watermelon seedlings bit the dust, but the other one is doing nicely. I put them into a hoop house covered with white fabric when it's really sunny, I think the direct sun might have been a bit too much for the little guys. – Catsunami Jun 19 '18 at 19:38
  • Yes, any seedling started indoors needs gradual hardening. Good luck. – InColorado Jun 20 '18 at 20:28
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If your last frost just barely happened, it's probably still too cold for the plant to be growing very much, especially if you have an insulated soil type (like clay or clay loam). However, the temperatures you mentioned aren't terribly cold (I'd be surprised if it were that warm every day, soon after the last frost, though); but they're not as warm as is optimal, though. Just be patient. Your plant should be fine (if it doesn't get much colder very often). I do believe fertilizer could help.

If you can put some black plastic on the ground around it, that can warm up the soil. Just cut a hole in the plastic where the plant would be, and put it over. Even a black garbage bag repurposed can be effective. I'm using black plastic for my watermelons, tomatoes and such, this year, and it seems to make a difference. I can feel the heat coming from under the plastic.

I tried the garbage bag on some tomatoes that we didn't have enough black plastic for, and they perked up quite a bit (they had been without black plastic for a while).

Be sure to water under the plastic, too (not just at the base of the plant).

Transplanting too early can hurt watermelon plants, even if it doesn't frost, perhaps especially if the plants aren't very old. Also, don't transplant too deeply. In my experience, the plants grow faster and survive more if you just plant them almost at the same level as they were before. Just put a little soil over the top of your seed-starting media. You don't want soil getting on the foliage: it can stunt plants if you don't wash it off. Also, soil being too near the growth tips seems to slow growth.

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    Thanks for the response, although I made the post 3 weeks ago, so it's a bit irrelevant by this point. It wasn't the last frost itself (warm spring), but the average date of last frost. Nevertheless, the watermelon that has survived the transplantation has been thriving in a hoop house. I started a few other watermelons in the greenhouse in a large pot and will put them outside when they're big. Also regarding planting depth - tomatoes actually like to be planted deeper as they produce roots along the stem. – Catsunami Jun 20 '18 at 16:49
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    That's a nice example of how plants differ, and how one has to learn the idiosyncrasies of each. Tomatoes do. Cucurbits don't. – InColorado Jun 20 '18 at 20:44
  • It is true that tomatoes produce more roots along a buried stem (although burying it wide is what people normally recommend over deep). However, if there isn't enough of the plant above ground, it still grows more slowly initially, in my experience, in my garden. But yeah, I meant watermelons for that precise depth I mentioned in my answer (just make sure they get enough water, since they need to grow deeper roots still). I didn't try the garbage bag on watermelons (but I have tried black plastic on them); that's why I mentioned tomatoes. – Shule Jun 20 '18 at 23:25

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