Answer: You didn't say what type of plants so I can't really give you accurate advice. As a general guideline, plant annual flowers and vegetables outdoors after the last frost date in your area. May 12 is the last date for the Marquette area; April 24 is the last date for the Detroit area. Check with your County Cooperative Extension office, local weather station or garden club for a more precise date for where you live. Again, without knowing what type of plants, I'm not sure what their soil needs or sun exposure requirements are. It's a good idea to "harden off" young seedlings.
Harden them off by setting outside for a few hours per day in a sheltered location, gradually increasing the time over a period of 7-10 days. This helps reduce transplant shock.
I've included a variety of information below on improving the soil and fertilizing during the growing season. Vegetable crops and annual flowers are heavy "feeders" and no matter how fertile the soil, it must be continually improved to maintain it.
In sandy soils, compost improves soil fertility, water and nutrient retention. In clay soils, it improves drainage. Add a 2-6 inch layer of compost and incorporate it about 12-18 inches deep. Continue to add lots of organic matter each year, which over time will not only improve your soil's fertility and drainage, but will also increase its ability to retain moisture and nutrients. It also provides food for earthworms and microorganisms that do the soil-building process. You can never add too much compost! You may want to incorporate a balanced fertilizer (e.g., 10-10-10) or add organic fertilizers such as fish emulsion, bone meal, and seaweed/kelp before the initial planting. Follow package instructions. If you prefer organic fertilizers, you may need to use three different sources, since they seldom come mixed together the way non-organic fertilizers do.
Side dressings of fertilizer are often beneficial during the growing season, but you shouldn't have to fertilize as frequently as you water. Perhaps once every two weeks at most. As your soil fertility improves, this won't be needed. Examine your plants to see what might be deficient. Slow growth and/or yellowing leaves is often a sign of lack of nitrogen. No flowers or fruit set means phosphoros is missing. Always ensure that the soil is moist before fertilizing, and then water the fertilizer in well afterwards. This helps prevent "burn." If you use a granular fertilizer, scratch it into the soil at least 4 inches to the side of the plant to prevent burning roots.
Here's a little background on fertilizers for your info: You probably noticed that fertilizers have 3 numbers on the container. These numbers refer to the percentage of nitrogen (N), phosphorus (P), and potassium (K) in the fertilizer. These 3 elements are referred to as macronutrients because plants need them in fairly large (i.e., macro) amounts to thrive. How these elements interact is complicated but in general terms, nitrogen produces lush green growth, phosphorous helps strengthen stems and produce flowers (and eventually fruit), and potassium keeps the root system healthy.
Here are some organic sources of nutrients:
Nitrogen: alfalfa meal, blood meal, coffee grounds, cottonseed meal, fish emulsion, seabird guano.
Phosphorous: bone meal, rock phosphate
Potassium: greensand, seaweed, kelp
After planting, add a 1-2 inch layer of mulch. Mulch is great to help retain soil moisture, reduce weeds, and as it breaks down it provides nutrients to the soil. Any organic matter can be used as mulch. Try compost, bark, wood chips, straw, or pine needles.
Good luck with your propagating!
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