Cucumbers, zucchini and squash are fast growing plants. You do NOT have to start them indoors as they can grow quickly right out of your garden as long as the soil and temperatures are warm. Some of the reasons to start them indoors is to improve germination rates or to get an established plant in the garden if you battle garden pests. Sometimes a bigger transplant takes hold better. And you can start them early, of course, to get a jump on the season.
If you want to start cucumbers, zucchini and squash indoors to get a jump on the season, you really have to make sure you start them in larger containers. You don't want them to get root bound and the small starting cells are just to small for bigger seeds and plants.
And (a big and) you have to time the seed start about 4 to 5 weeks before the soil temperature is in the upper 50's and you are getting day temperatures around 70 degrees. Holding these plants to long in containers will get you transplants that are small and stunted. The stunted plants will take several weeks to recover and grow. You will have lost the 4 weeks you were using to get a jump on the season.
They are heavy feeders and you will see them grow quickly indoors. I recommend feeding them with a 1/2 strength liquid fertilizer at 2 or 3 weeks when you see the first true leaf get to size. And you can do it again the week before they would go outside. A starting mix with a low amount of fertilizer won't hurt. You should still do the liquid feedings. The key is keeping fertilizers for indoor plants at half strength. There is no rain to move concentrating fertilizers away from your plant's root. They just don't need full strength indoors.
Good Luck in Your Gardens
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