Transplanting tender seedlings outside is a regionally specific farm or garden task. In zones 8-11 you may be able to transplant seasonally appropriate plants all year round. For colder growing zones you may need to transplant your seedlings all at once in the late Spring once nighttime temperatures are reliably above 50℉. If you will be growing in a hoop house or cold frame you can move up your planting time by a number of weeks.
The best time to transplant any plant seedlings, whether it is one you have purchased from a local nursery or started yourself in seed starting trays, is once it has reached the proper stage of growth and outside conditions are appropriate.
AT WHAT STAGE OF GROWTH DO YOU TRANSPLANT SEEDLINGS?
Waiting until the seedling develops its second set of true leaves is one way to determine if a seedling is ready or not - but that is not always a hard and fast rule. Seedlings grown in cell traysneed a sufficient root system in place before transplanting outdoors or into a cold frame. If the roots have not developed enough before you attempt to transplant outdoors, the root ball may fall apart when removed from the tray. This can break the fragile new roots and stunt the growth of your plants.
Having a strong root system is the best indication. You can check for proper optimal root growth by grasping one of your seedlings at the soil surface and gently pulling it from the tray. The entire plug should slide out of the cell tray. If the seedling is immature it will pull out and leave the majority of the potting soil behind in the tray.
If the seedling is overdeveloped it will come out easily but be root bound. Root binding happens when seedlings are left in plastic pots or trays for too long and the roots encircle the entire plug. Root bound seedlings are easy to identify because there will be numerous roots sticking out of the drainage holes and when you remove the plug you will see mostly roots with little growing media visible.
How to get seedlings with optimal root growth for transplanting.
The three best ways to achieve a healthy root system when growing your own plants from seed are:
Use a bottom watering system to avoid compacting the soil.
Plant in properly sized trays that encourage downward root growth.
Pot up your seedlings before they become root bound if you cannot transplant them yet.
How do I choose the best seedlings at the nursery?
When choosing seedlings from a nursery to transplant into your home garden there are a few things to look for to ensure you get the healthiest plant starts you can for a successful garden. You want big and healthy stems and leaves but not so big that the plant has overgrown its small pot and become root bound.
Avoid leggy plants with tall thin stems. These have likely not been receiving direct sunlight and will possibly fall over when planted outside.
Choose smaller, compact seedlings. They are ready to grow big once you plant them.
Check the foliage for discoloration and damage. Leaves should be uniformly green (unless it is a plant with variegated foliage).
Look at the underside of the leaves to be sure there are no hitchhiking pests like aphids or insect eggs.
Check the plants for signs of disease including yellowing of the leaves, brown spots and dried out tips.
Look at the bottom drainage holes, a few visible roots are ok but if there are a lot sticking out through the holes it is probably root bound.
Look for moss or signs of fuzzy mold on the soil surface. These are likely signs that the seedlings are old stock or of overwatering both of which which can cause weak transplants.
Squeeze the sides of the pot. It should give a little under pressure showing that there is still some loose potting soil, meaning the roots have a little space left to grow into.
Too little soil moisture is just as problematic as too much so make sure the potting mix is moist but not sopping wet. Root bound seedlings will often become hydrophobic and not take up water properly.
Choose plants that have only leaves and possibly buds. Vegetables and fruits that have already begun to flower and fruit will stay small even if they are given more space.
Should I fertilize my seedlings before I plant them?
Most seedlings will grow just fine without additional feeding. In fact, too much nutrition in the growing medium can cause seedlings to be weak or prevent seeds from germinating. Using a balanced potting mix that contains some aged compost or worm castings will provide your plants with everything they need while they are small.
If you are using a sterile seed starting mix like ProMix that contains only coco coir, peat moss or perlite to germinate your seeds they may require a very light feeding once they have a second set of leaves. This only applies if you will be continuing to grow them indoors for a while beyond the second set of leaves.
We generally don't like to give direct advice about fertilizing as it is different for every individual and situation, but buying a well rounded potting soil will help to ensure that your seedlings are covered nutritionally. If you are planning on potting up things like tomatoes, peppers, cucumbers or squash and have started them in a sterile seed starting mix you can then pot up into a larger container with a balanced potting soil. This is more effective than using garden soil which may not have a balanced nutrient profile.
The length of time a seedling can stay in a cell tray depends on the size of the cells. Deeper and larger cells both allow for extended growth as the plant has more room. Seedlings otherwise can become root-bound if not given adequate space for the roots. Typically, after sowing the seeds, the cell trays are used for around 3-4 weeks before transplanting occurs - whether it be to an outdoor plot or into a larger container. If transplanting tender seedlings outdoors, you may want to consider the use of a frost blanket to protect from late Spring frosts. Learn more about frost blanket in Frost Blanket: How to Use it and When.
One thing to keep in mind with transplants, is to choose your sizing according to how long the transplant must stay indoors before getting transplanted into the garden. This will factor into Selecting the Right Cell Trayfor the seed types you will be growing.
HOW DO YOU TRANSPLANT SMALL SEEDLINGS?
Farmers have moved into using cell trays in order to increase the amount of transplants that can be grown in a smaller space. Many have their own tips and tricks to finessing a seedling plug out of a cell or using a plug popper. Bootstrap Farmer cell trays are specially designed for growers by growers to make this chore a bit easier. Here's a great video highlighting how to remove the plugs from the cell trays.
Why plant seedlings over direct sowing seeds in the garden?
Sowing seeds into cell trays and planting starts into the ground offer some major advantages to the grower. You will be able to get a head start on the growing season by starting weeks before your last frost date. Planting into cell trays also helps increase the number of seedlings that can be grown in a smaller amount of space. This is a huge advantage for growers who are growing for quantity.
Another advantage of growing out seedlings to transplant is a higher seed success rate. You only transplant strong seedlings, therefore generally are able to have more growing success. Want to learn more about the process of starting your very own seedlings this season? Read Seed Starting 101 for everything you need to know about starting vegetables, flowers, and herbs from seed for your garden.
Be sure to share your seed starting successes with us on Instagramand tag us with questions you have about the process.
Planting season is an exciting time for growers. But, before you get too anxious about planting those seeds, have you cleaned your equipment since the last growing season? Read about the BEST methods to clean and care for your propagation trays.