How cold is it supposed to be? That's the first thing I'd consider. And, by the way, I am very cautious about trusting the forecast overnight low at this time of the year as we often drop lower (sometimes much lower) than forecast. Lately, we're averaging an overnight low at our house that is running at least 3 degrees lower than forecast and sometimes 7 or 8, so when I look at the NWS point forecast for my specific area, I mentally subtract at least 3 degrees. That means that my current forecast low of 36 for Saturday night might end up giving us a low of 33 degrees, and I'll cover up warm-season plants accordingly. So, look at your forecast low and then, if it is pretty close to freezing, realize your actual temperature might be a bit colder than that (or, a bit warmer, though that is more rare at this time of the year).

With any plants, conditioning plays a role in how well they tolerate cold temperatures. Cool-season plants can tolerate different degrees of coldness, but it might be a bit of a concern if your cool-season peas haven't had any exposure to cold weather at all in their young lives because that means they might not be conditioned to tolerate the cold. Normally peas can tolerate temperatures well down into the 20s, but that normally is the weather they are exposed to once planted, and this year we just haven't been cold. So, based on that, if you are generally a cautious person, you might not want to put them in the ground until the cold night or nights have passed.

However, there's other options. You can go ahead and plant them, make sure they are watered in well (soil should be moist not sopping wet), mulch around them to help hold in whatever heat the ground currently possesses, and then gently cover them up in advance of the cold night or nights you're expecting. You can use something lightweight like an old bed sheet or an old sheer curtain. You don't want something with so much weight (like a heavy blanket) that it will crush the plants and break them. This is probably what I would do. My peas have tolerated temperatures as low as 26 degrees since I planted them, but I did cover them up on their first cold night because I knew they weren't hardened off to cold temperatures at that point since I'd just transplanted them into the ground. Since then, they have been exposed to a wide range of temperatures and I'm not going to cover them up this weekend because they don't need it. They've been in the ground around 3 weeks, I think. You can go right ahead and plant the seeds now too. They'll be underground and not harmed by the cold temperatures since soil temperatures are already astonishingly warm for mid-March.

Nasturtiums are cool-season plants that often survive fairly deeply into the summer here, but they are not cold-hardy. They can suffer freeze injury damage at 32 degrees and if you happen to get a hard freeze or a frost, they can die. Sometimes at below-freezing tempertatures they will die back to the ground and regrow, but sometimes they freeze back and don't regrow. You could plant them if you believe you can cover them up well enough to keep them from freezing or you could hold them another day or two or three. Your choice. I put all my nasturtiums in the ground last week and I will cover them up here on Saturday night. I never plant any warm-season plants ahead of my average last frost date unless I am willing to cover them up when frost or freezing/sub-freezing temperatures threaten. Keep in mind that your garden can get frost or patchy frost at temperatures in the 30s but above 32 degrees. I sometimes see frost when we are as warm as 38 degrees. Frost does not always kill sensitive plants, but it often damages them if it stays on them long enough. Three minutes of frost or even 30 minutes might not harm a plant, but a few hours of frost might. There's so many variables involved that I don't take any chances and just cover up the plants with a textile (not a plastic unless it is suspended far enough above the plants that it is not touching them. Any part of the plant touched by the plastic can freeze).

My average last freeze date is March 28th, so I expect to cover up all my warm-season plants on Saturday night, which will be our coldest night of the week. Some years, we keep getting a recurring cold night here and there every couple of weeks, and had a long string of years where our last freezing night hit us here the first week in May, but that didn't happen last year or even the year before. Because I'm in a low-lying microclimate and have lost plants (and one year lost most of the garden on a night when the forecast low was 50 and the actual low was 32) on some late cold nights, I'd rather just cover up the plants than take a chance on losing them. I understand my microclimate pretty well and usually can tell if the late afternoon/early evening temperatures are dropping quickly enough to worry me. More than once I've run out just before dark and tossed row covers over a few rows because I felt like the temps were going to fall dangerously low for those plants. It's always better to be safe than sorry. This Saturday, I won't be running out at the last minute and covering up the plants. I'll be doing it Saturday afternoon when the ground is still warm and I'm not rushed.

If you're in a warm microclimate and your plants are fairly well-sheltered, then you probably don't have to worry about an occasional night dropping too cold or too much lower than forecast, but if you haven't gardened in your location long enough to know what your microclimate usually is/does, then it is better to either hold off on planting or to plan on covering up your newly planted plants when that occasional low temperature near freezing is forecast. Had your peas regularly been exposed to pretty cold weather since they sprouted, the kind of temperatures forecast for this weekend wouldn't bother them at all, but it might set back peas that have had no real cold exposure. A warm late winter/early spring is hard on cool-season plants as they often get more exposure than they like to warm weather early in their lives and not enough exposure to cold weather to harden them, and that's exactly the situation you're facing. If in doubt, cover them up on those cold nights if they're in the ground. Otherwise, you're likely to toss and turn and worry about them and not get a good night's sleep.