Hi Lucie, welcome. I don't know where you are, but you mention putting in seedlings later, when the weather warms up--so I suppose you are somewhere in a northern or cooler area, is that correct?
I have been planting polyculture mixes for several years. Usually I just sow a mixture of whatever cool-weather seeds I have, as early as possible. Some of my beds have frames with pieces of PVC so I can erect tunnels, and plant early. I planted two beds in early April. the one under plastic was actually planted later, after I finished preparing the bed. The seedlings coming up there are already as big as the earlier planting on a bed with no cover, just for comparison.
I use the instructions from gaia's Garden
, by Toby Hemenway
, and sow a mix of mustard, cress, arugula, lettuces, peas, onions, kale, chard, spinach, beets, carrots, radishes, dill, and other cold tolerant crops. He says to sow fairly thinly, about 1-2 seeds every inch (I usually plant 1 presprouting pea seed
every foot or so, and one onion set every foot or so). Then, when the seedlings start to grow, thin them progressively so as to open up space for the remaining plants. Then, after the weather and soil warm up, plant bean seeds and transplant cabbage, broccoli, basil, etc into the spaces opened by harvesting the early greens. He doesn't recommend putting tomatoes in a bed like this as they tend to be big, sprawling plants, but it could work if you install a trellis
on the north side of the bed and train the tomatoes on the trellis.
The idea behind the mustards etc is for them to serve as a kind of living groundcover. They come up early, before the slower germinating things like carrots, and can be thinned out for salads or stirfries as the other plants start to need more space. I like to let a few go to seed and use the flowers and radish pods in stirfries as well.
Strawberries seem to be one that works better in a bed of mostly perennials, or under fruit trees
, etc, and not so good in beds of annuals, because of soil disturbance, and the maintenance, mulching etc that might be a problem with the salad crops.
Depending on your weather and climate, you could plant the cool weather crops now and add the warm weather ones in a few weeks, when the weather is more settled. Hope this helps and it works for you. I love my polyculture plantings. They seem to work a lot better, to get a variety of food over a longer season. Just keep thinning and harvesting those that are ready.