Growing seedlings in Denver

Some growing season data and recommendations for Denver area:
Some plants can stand a small frost, some can’t. To be safe with plants that can’t stand any frost, it is best to wait a week or two after the “average last frost” to be sure your plants don’t break your heart and freeze before they even get started. If there is some weird weather (there usually is in Denver), you can cover your plants with old bed sheets or row cover to protect them about 2-4 degrees Fahrenheit. If you put jugs of hot water under these blankets, it usually protects nearby plants an additional 2-4 degrees for a total of 4-8 degree protection.

Average last frost: April 30th-May 5th
Average first frost: Oct. 1st – Oct. 6th
Average length of growing season: about 150 days

The first thing we will need to do is dry and store seeds from the previous year, or buy some seeds to plant.

So, we have seeds, the next thing to do is to develop a gardening plan. This will tell us where we want to plant certain vegetables or flowers. Making a map will also give us a good idea of how many seeds we should start for each area. This map could be as simple as drawing a map of your garden and labeling where you are going to plant certain seeds. The map could also be fairly complex. I try to get at least two planting cycles per garden bed. Some people get three cycles in a season. This means that we will have two or three maps per year per bed. If we companion plant, then you will have at least two plants in a section at a time, which makes the detail of your map even finer.

I keep this map for three years at least. It is important to track where certain plants were planted. For example, we would not want to plant potatoes where tomatoes were planted last year. I am extra careful and won’t plant potatoes even two years later. This is because the plants have diseases in common and they also take similar nutrients in large amounts from the soil.

But don’t worry about this too much if you haven’t been tracking this closely. Plants, especially most vegetable plants are resilient. If you happen to forget to make a map, or move a bed and can’t keep track of what used to be there, the plants will probably be fine. But it is a good practice to get into. Record keeping is an underestimated skill of good gardeners.

If we are planting directly into the soil, I recommend following the directions on the packet of seeds. If you don’t have a packet or it doesn’t tell you some information, you can usually find this information online from seed companies. For planting depth, a good guide is to plant a seed between 2-3 times the longest dimension of the seed you are planting. For spacing, it is OK to plant the seeds a little bit closer together than you think the plants need. You can always come back and thin, but it is impossible to plant more seeds later and expect them to catch up with the other plants.

But since we are here in March, well before most outdoor planting starts, we will mostly deal with planting seeds which will later be transplanted outside. So far we have seeds and a map, now we need some soil-like substance to start your seeds in. The easiest thing to use is a seed starting mix. There are many good products out there. What I have been using for a year now with excellent results is Waste Farmer’s seed starting mix which is all vegetable compost with some biochar and maybe a couple of other things. It is almost if not entirely made from materials in Colorado and mostly from the Denver area.

The seed starting mix should be fluffy and hold moisture well. Seeds need many things in order to sprout and what they need from the soil is air and water and a small amount of nutrients.

Now we need some containers to put the seed starting mix into. I use little molded plastic pots like you would buy plants from the store in. These stack into each other and store well until the next season. If they are handled gently, they can last several years. But if you want to, you can use anything that will hold at least an inch and a half to two inches of soil. My mom swears by planting tomatoes in broken egg shells held in place inside of an egg carton. I have used old milk jugs and yogurt containers. If you are re-using plastic containers, make sure that you wash it thoroughly before filling it with dirt. Just make sure that any container you use has some holes in the bottom for drainage. The seeds need water and air in the seed starting mix and allowing drainage is what allows air to get around the seed.

This also means that we will want something underneath all of these containers to catch the excess water. I also use molded plastic trays like you would see at a nursery, except that I use trays that have no holes in them. This way, I can water my plants from the bottom up. A common problem with starting seeds is that watering from above can dislodge the seeds from the soil and they will float to the top, or worse, they will spill out of the container that they are in. This only happens occasionally, but this is a way to guard against it. But if you water carefully with a sprayer or even with a watering can, our seeds should be fine.

Next we need to find a source of light. Most plants want at least 8 hours of light to grow healthy, vigorous plants. If you have a window that gets plenty of light, you might want to set up a table in front of this window. If your best window doesn’t get 8 hours of light, you can supplement in the mornings of evenings (or both) by putting a light over the seedlings. You can just turn the light on and off manually or get a timer to turn the light on and off at specific times.

I have never found good information on which types of light bulbs work best for lights. There are grow lights you can buy at a grow store which are hundreds of dollars and these definitely work, but for vegetables and flowers and normal plants, I use fluorescent bulbs that say they produce “natural” light. I also have known people to use halogen bulbs and incandescent bulbs with pretty good success. The fluorescent bulbs are much more efficient than halogen or incandescent bulbs. They are much less expensive than the grow lights I have seen.

Next, most of the sorts of plants that we all want to start inside for transplanting are warm season plants such as tomatoes, peppers, eggplants and cucumbers. So, they will need a warm environment to encourage the seeds to grow. These plants really want their seeds to be at least 70 degrees Fahrenheit. The seed packets for most tomatoes say to keep them between 70-80 degrees. But if you are cheap like I am, you don’t want to heat your house up this much all spring time long. I keep my house above 60 with a max of 65 degrees and have not had any problem getting plants to come up.

If you want to provide extra heat, you can invest in electric heating pads to go underneath the plants. These can be purchased from a gardening store or online. I have even known people to buy electric heating pads from Walgreens to put under their plants. If you choose this option, you will have to pay careful attention to how warm the pad gets because these are meant to heat up more than a plant would want to be. You can remove some of the heat by putting something in between the pad and you your plants, like a couple of sheets of cardboard or an old towel or something that will create space between the heater and the plants. Also, these heaters almost always come with an automatic shut off after a certain period of time (to keep people from burning themselves when they fall asleep with a heating pad on them). If you are using it for plants, you probably want one that doesn’t have this automatic turn-off.

Now you just have to continue providing warmth, light and water until they start sprouting. The main problem is under and over watering. The soil should always feel moist, but you should not be able to see liquid in the soil. Of course, you will see some liquid right after watering, but you should not be able to see liquid in the soil even 10 minutes after watering.

Having a fan gently blowing across the plants is not a bad idea. Tomatoes, in particular, will not grow strong stems unless they are moved a little bit. Some people gently pet their plants every couple of days, but setting a fan up to blow on them every once in a while is an easy way to do this also. The plants will grow up to have much stronger stems and be more capable of transitioning to the outdoors.

Plants do not like sudden changes, so check on your plants often. If they dry out too much and then you flood them, they may not come back from this double catastrophe. So, check on them at least once a day.

Plants can be successfully transplanted outside once they have true leaves. True leaves are the leaves that grow after the cotyledons (the first two or sometimes one leaf that come up from the soil are cotyledons). All leaves grown after these first leaves will be “true” leaves. It has been my experience that plants which have more than two fully developed true leaves perform better in transplanting than plants with two or less true leaves.

When you go to plant them outside, this will be a big change for the plants, so it is recommended that you put the little seedlings outside for some morning sun a couple of days. Gradually lengthen the amount of time the seedlings spend outside until you feel that it will not be such a shock for them to be outside. I usually do this over a 4 day period adding about 3 hours each day. Some people do it in 3 days and some people take a whole week of gradually acclimating their plants to the out doors.

It is good general practice to plant annuals slightly deeper than they were before, this will make them more likely to get a good water supply since this makes their roots deeper. Somewhere between a half and a whole inch is a good generalization.

The last thing to go over is how to plant them in your garden. Annuals, unlike perennials, can be planted deeper in the ground than they were when they were inside your house. If you have a really “leggy” plant that you are transplanting outside, you can plant it as deep as you need to so that just a couple of leaves are sticking out the top of the soil. Most annuals can develop roots where there used to be leaves or stems.

When you first plant them, you should water them gently, but thoroughly; the soil around their roots should be kept moist for about a week after planting. Gradually wean them over this first week. After this, you can start treating these plants like any other plant in your garden. Some people also use some light, balanced fertilizer with their first couple of waterings to encourage the roots of the seedlings to move out into the garden soil.

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