The short answer to your question is that in Baltimore, the last frost date is March 27 with a 30% chance of frost after that date (Farmers Almanac) and April 22, with a 10% chance that a frost can still occur. The more important issue is soil temperature...see below. 

Spring Planting Wildflowers in Colder Climates

When planting seeds in cold climates that experience snowfall and freezing temperatures, you must wait until after all threat of frost has passed. Even after warm weather arrives in spring, there is still a lingering danger of late spring frosts that can kill off freshly-sprouted seeds and young plants.

The greatest threat to spring-planted wildflowers in cold climates is late spring frosts that can kill tender young seedlings. To avoid the risk of a cold snap harming your planting, plant after the last spring frost date chart for your area. Even after the risk of frosts has passed for the season, it's best to wait for the soil and air to warm up enough to provide favorable conditions for your plants to grow. For fast growth and strong, healthy plants, plant when your soil temperature reaches a minimum of 55F, or as a rule of thumb, about the time that tomato plants are put outside.

It will be important to keep your seeds and soil well-watered for the first 4-6 weeks.

In cool areas, soil temperatures are just warming up to the right temperature (55F) to germinate seeds.

Prepare Your Site For Seeding Wildflowers

First, thoughtfully choose a site for your wildflower planting. Wildflowers can be great for solving problems on your site. Then, prepare your site - better preparation = more flowers!

Tips For Choosing A Site For Your Wildflower Planting

Full sun is a must for most wildflower varieties. Choose a sunny spot with 6+ hours of sun. For areas with 4-6 hours of sun, get a partial sun mixture.

Your soil is probably already perfect for wildflowers! Unless your soil is actually sterile, which is rare, it's recommended that you use your soil just as you find it. If yours is heavy clay, you can till in sand or peat moss to loosen it. If it's sandy, you can till in humus or compost to make it more moisture-retentive. The test is simple: If anything is growing in the area — even if it's just grasses or weeds — the area should support wildflowers without concern.

Wildflowers do not need fertilizer to grow well. Wildflowers, as we see on every roadside, are extremely adaptable and do well in poor soils.

The only absolute requirement is good drainage. Choose a place where water does not stand for longer than one hour after a rainfall.

Prepare Your Site: Better Soil Prep = More Flowers!

Preparing the planting area is a task that you do not want to overlook; the more time you spend prepping the area before seeding, the better results you’ll have.

Clear weeds, grasses, and other plants (roots and all) to make room for your wildflowers to grow and thrive. There’s a set amount of water, nutrients, and sunlight available in every planting space. By leaving other plants in place before sowing your seeds, you'll risk creating a competitive environment where your wildflowers will be stressed as they fight for resources. Crowding and competition can cause leggy growth (extra-long, floppy stems) and weak plants, which jeopardizes the lifelong health of your planting. We don't recommend just throwing the seed out in the field; anyone who’s tried scattering seed without removing other plants has been sorely disappointed when their wildflowers don’t come up.

The better you prep the area, the more easily two very important things will happen:

Your seeds will germinate quicker and stronger without competing plants shading them out and ‘stealing’ available food and water.

Without the stress of competition early on, your young wildflowers will be better suited to compete with weeds and grasses that might try to grow back.

Ways To Clear The Ground Before Planting Wildflowers

While you have a few choices to consider for how to clear your soil, there are two factors that will help you to decide which one is the best for you: size and lead time.

Size: Large spaces are more easily prepared with equipment like rototillers or tractors, whereas hand tools will be just fine for prepping small gardens and containers.

Lead Time: With a few weeks, months, or even an entire season ahead of your planting date, you may be able to prep your soil using labor-saving, cost-effective, and eco-friendly methods. Here are some soil prep approaches that work with different schedules:

Planting Immediately: If you're looking to sow your wildflower seeds within a week's time, use rototilling or hand tools to remove plant growth and existing roots. Some people rent or borrow equipment if they don't own it, while others are happy to prep their soil by hand to keep their planting budget-friendly.

Planting in 3 months: If you have a few months ahead of you, you can make use of natural herbicides and weed killers. This approach reduces physical labor, and also allows time for the chemicals to dissipate before they can do any harm to your wildflower planting. This time frame also allows you to work the soil with a tiller or hand tools multiple times, and for weed seeds to be repeatedly brought to the top of the soil and killed off, diminishing their overall appearance in your meadow.

When and How to Scatter Wildflower Seed Over Your Site

Average Soil Temperatures

As mentioned above, it's best to plant after the chance of late-spring frosts in cold climates, or before the scalding summer sun arrives in hot climates. Seeds will germinate when your soil temperature is 55°F or warmer on average. Air temperatures often warm up before soil temperatures do. One of the most common mistakes people make is to sow seeds when the air has warmed up but the soil is still cool.

Working With Your Schedule & The Weather

Once your ground is bare and loose, you'll want to choose a nearly windless day for your planting so that seeds stay put where you'd like your plants to grow. While high winds and driving rains should be avoided (they can easily wash your planting away), regular rainy days are a perfect way to care for your plantings without needing to reach for the hose.

How Much Seed Do I Need?

More seed does not always mean more blooms! While it may be very tempting to throw extra seed down, this usually brings the opposite effect you were looking for. Seeds sown too densely can create competition among seedlings, causing them to become leggy or strangle one another out.

How To Sow Wildflower Seed: The Simple Split & Sand Method

There are two good reasons for the Split & Sand Method: First, the sand helps you spread it more evenly. Second, since it is lighter than the soil, you'll be able to see where you've sown seeds. The Split & Sand Method, along with some time spent practicing, really helps with an even application.

The Split & Sand Method

Separate the seed you're planting, no matter the amount, into roughly two equal parts.

Put the first half in a clean bucket (or coffee can, or anything else handy)

Then add in roughly eight parts of dry sand to one part of seed, and mix well. (For example: 8 cups sand to 1 cup seed.) Always make sure that your sand is dry, especially if it has been stored outdoors. Wet sand has a tendency to clump and can cause your seed to be applied unevenly. If possible, starting with new sand helps prevent contamination.

Test out your sowing technique. Your goal is to lay your seed down as evenly as possible, and you're likely to be surprised by how quickly it leaves your hand or the spreader.

For the best chance of an even application, scatter your seeds in two sowings. Take the container with one half of your seeds and sow them as evenly as possible while walking across your site from north to south.

Then take the other half and apply in a similar manner, this time walking in the opposite direction.

Compress Your Wildflower Seeds Into The Soil

After you’ve scattered your seed, it’s very important to compress your seeds to make contact with the soil. Good seed-to-soil contact helps to speed up germination, ensures that moisture and nutrients make their way to your seeds, prevents wind, water, and natural occurrences from moving seeds around, and helps to anchor your wildflowers' root systems in a good spot.

For small-sized patches, you can use your feet to compress seeds into the soil– either bare foot or in shoes.

For medium-sized gardens and beds, lay a piece of cardboard or plywood over the soil and walk all over it. This will evenly distribute your weight across the soil.

Watering Your Wildflower Planting Is Essential!

After you've planted, if possible, give your seeds a thorough, gentle soaking. The soil, wildflower seeds, and seedlings must stay moist until they are 4 - 6 inches tall, which typically takes 4 - 6 weeks. Since wildflower seeds are not buried beneath wet soil, they have constant exposure to the sun, so they'll need your help staying moist!

Watch Growth & Blooms Appear

Now the fun part begins! Within 1-3 weeks, depending upon the varieties you've planted and your growing conditions, you'll begin to see growth appear. Blooms themselves follow more unique schedules: Annual wildflower blooms appear within 6-12 weeks. Most Perennials require a full season of growth in their root systems before coming into flower in their second year and successive seasons.

A question that arises often when gardeners are looking at their planting site in spring: “Are those wildflowers or weeds?” Many young seedlings are hard to identify. We have 3 pieces of advice for you:

Grow a wildflower cheat sheet! Plant some of the individual seeds from your mix, label them, and see how their foliage develops.

Get a wildflower identification book. This should give you information on many of the wildflowers that you’ve planted.

Learn about your local weeds – at every stage of growth. Most gardeners struggle with 5-10 aggressive weeds on their property. Getting to know what they look like as seedlings, adolescent, and adult plants will make it easier to spot them within your plantings – so you can pull them without mercy whenever you see them!

"Adopt the pace of nature: her secret is patience." - Ralph Waldo Emerson said it best. We've talked a lot of customers out of ripping up their wildflowers after suspecting that their planting sites were filled with young weeds. When in doubt - do not pull your plants. If you give your seedlings some time to grow, you're likely to find that they were wildflowers all along!